I suggest you take the time to play each of these games, as they're all available for free in some form, and they are generally very quick to jump in and play. Some do require download and install, though. Each of these games exhibits excellence in various areas, which I've attempted to highlight within the spoilers below them. A handful of the games also have some very nice gameplay videos you should check out as well.
Have fun!
OSMOS - Play!
+ Show Spoiler +
Tranquil and calm in its delivery and play, Osmos is the ultimate ambient experience. Check out the video below!
http://vimeo.com/2754925
SAMOROST 2 - Play!
+ Show Spoiler +
A very unique graphic adventure game which provides some really interesting puzzle design, creativity, and audio. The art is amazing and the puzzles are very mentally rewarding. Samorost 2 is bursting at the seams with humor and personality - it's a must-play. Even your grandmother could appreciate this game.
AUDITORIUM - Play!
+ Show Spoiler +
Auditorium is very clean and artsy in both its design and delivery. Direct the flow of music using various directional tools and complete the ambient tune. It really is a beautiful production.
POWDER GAME - Play!
+ Show Spoiler +
This is the ULTIMATE sandbox. Literally. It's a veritable playground for the mind. It will blow you away. Seriously, stop reading this right now and go play!
WORLD OF GOO - Play!
+ Show Spoiler +
This is perhaps the most clean-looking professional production out of this entire list, and provides some of the most interesting gameplay as well. The quality of this production is just simply off the charts. Check out the VERY COOL video below!
http://www.dailymotion.com/video/x72eqm_world-of-goo-trailer-2-directors-cu_videogames
BOOMSHINE - Play!
+ Show Spoiler +
Perhaps the simplest game on this list, but also very fun! The game is quite basic - Simply choose a dot, blow it up, and try to cause a massive chain reaction. A very quick, but fun, play.
AQUARIA - Play!
+ Show Spoiler +
This is an adventure game in which you swim and sing your way through a water wonderland. It features some very innovative controls and gameplay, along with some pretty impressive production values. It radiates a very pleasant experience overall. Check out the video below!
http://vimeo.com/2461697
INSANIQUARIUM - Play!
+ Show Spoiler +
This is a very unique game in which you care for an aquarium full of fish. Each fish produces currency which you use to feed your fish, grow your population, and win the level. It's a relatively unimpressive concept, however, there's some really great design within the various fish and creatures which inhabit the tank.
MIGHTIER - Play!
+ Show Spoiler +
The pen is mightier than the sword. Mightier is a game which brings a whole new meaning to user interaction. You actually solve puzzles on paper, load them into the game, and play! Check out the video below!
http://vimeo.com/2376591
STEAM BRIGADE - Play!
+ Show Spoiler +
I loved this game. Your goal: Destroy the opponent's factory before he destroys yours. The game features very thought-provoking 2D RTS style gameplay. There are indeed games similar to this, but none of which have the amount of polish and innovation as Steam Brigade. Apart from games of similar style, the player is granted an avatar which provides a direct connection to the beautiful game world. So many mechanics inside of this game are so unique and interesting, that if I began to list them all this would end up being a page long. On top of the already awesome gameplay, the art team behind this game managed to create an absolutely AMAZING art-style behind the very charming steampunk theme. This game is just all-around great!
THE GRAVEYARD - Play!
+ Show Spoiler +
The Graveyard is more of an (slightly) interactive short film, rather than a game. However, unlike a short film, The Graveyard manages to successfully grab emotion from the player through interactivity. This game takes only around 10 minutes to play and displays some of the better experimental game design I've seen. Beware: This isn't so much a game as it is an experiment in game design and human emotion. If you're not entirely familiar with what it's trying to accomplish, try playing it by yourself in a quiet room, and allow your mind to go wild. After you're done, try imagining how much 'worse' the experience would be from the perspective of someone watching the game be played, rather than playing the game.
MOTHERLODE - Play!
+ Show Spoiler +
Motherlode is an all time favorite of mine. X-Gen Studios is a great game developer, and this is by far their greatest creation. The art is acceptable, the story sucks, and the music is okay, but what really makes this game appealing is the innovative gameplay. Motherlode is a fantastic example of quality game design trumping all other aspects of production.
KNYTT STORIES - Play!
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An excellent shell for creativity, Knytt Stories provides the foundation of some very cool gameplay elements, and then allows for the creation of some really interesting levels. There's a simple tutorial and sample level (which is really great!) that quickly describe each aspect of gameplay in a very streamlined fashion. After completing these, you can get more levels through official packs, other players, or you can create one of your own!
CAVE STORY - Play!
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From what I've played, I can tell you that Cave Story is a brilliant game. It's an action-adventure game in which you'd expect to find on your NES, or SNES console, but this isn't just any old action-adventure title. It's deep, innovative, creative, and charming. The game was entirely designed and created over a period of FIVE years by a single person. The game has since been translated into English, and is actually being enhanced and re-released on Wii-Ware sometime this year. A well deserved reward for this man's awesome work. (Note - Be sure to download and install the English translation patch if you so wish.)
THE ENDLESS FOREST - Play!
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Brought to you by the same team who created The Graveyard, The Endless Forest is yet another experiment in game design. A social interaction experiment on a grand scale. It's a massively multiplayer game in which chatting doesn't exist. All interactions must be expressed through your deer avatar's actions. There are no goals or standards, just explore and see where the game takes you. On top of all this, you can set The Endless Forest as your screen saver! How cool is that!?
COIL - Play!
+ Show Spoiler +
I'm not really sure what to think of coil. It's a very interpretive artsy game. The gameplay consists of various stages, stages of conception. Each of these stages has some very open-ended narrative between them... Making you question what is actually going on... Human conception? Alien conception? FORCED conception? There's the possibility of some very taboo interpretations here, and that's what makes it interesting. I will honestly say that the gameplay is a little annoying, but utilizes interesting control, and is well worth it for the mind-engulfing narrative. The entire game lasts only around 5-10 minutes and consists of several different minigames.
AN UNTITLED STORY - Play!
+ Show Spoiler +
You are an egg. That is the tagline for this 2D adventure, and it's quite accurate. Roll your way through some very well-designed obstacles in order to upgrade your egg and explore the expansive world. An immensely neat platforming game.
THE MARRIAGE - Play!
+ Show Spoiler +
The Marriage is quite complicated and quite simple at the same time. It's also quite difficult to figure out, but very much well-worth the time it takes to do so. On the surface, the game is quite ugly and hollow, but in reality it's EXTREMELY deep in content. I suggest you read, and follow the advice of, the creator of this game. He'll let you know when you should stop reading and give the game a shot before reading on. Try to play the game and see what it means to you before reading deep into the creator's explanation. This is an extremely deep game which can be interpreted in various ways depending on who you are.
I WANNA BE THE GUY - Play!
+ Show Spoiler +
An extremely difficult game which embraces the past, and allows for great gaming reminiscence. I Wanna Be The Guy is absolutely hilarious. You must make your way through the NEAR-IMPOSSIBLE platforming levels that are thrown at you by using both excellent control and good problem solving skills. I'd estimate that 90%+ of people who play this game don't even make it past the first screen. It's a really decent production, and provides an excellent amount of entertainment value, even if you don't make it very far!
For each of these games, there's tons more that are just as worthy of being on this list. These are just simply my favorites out of those I have discovered, as of this moment. Feel free to post here or PM me if you think you've found a gem in the rough.
Thanks for looking.
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My Favorites:
1. World of Goo
2. Aquaria
3. Cave Story
4. Steam Brigade
5. I Wanna Be the Guy/Samorost 2/Powder Game
(Oh, and at least watch the trailer for Mightier, it's neat.)
They're all amazing though, and some are VERY quick plays, so check them all out if you get the chance...
For the 'less artsy' of you guys, or those of you who aren't horribly interested in experimental/interesting game deisgn, go ahead and skip the following:
1. Osmos
2. The Graveyard
3. The Endless Forest
4. Coil
5. The Marriage