Edit: this game is a very personal experience when you get started; you might move from room to room at a snail's pace, but as the world begins to materialise in your head you'll begin to speed up and get to grips with it and suddenly find yourself in a delightful and limitless new universe
This is a text-based "MMO" that is based (with perfect accuracy) on the best-selling Discworld series by Terry Pratchett.
The game has been developed for 20 years. It is the most detailed and carefully crafted game in existance and has one of the best skill/advancement systems you'll ever experience.
You navigate the game by reading a description of the room you're in and then interacting with commands such as "look", "get" and "wear".
The text-based nature of the game means that you are put into an extremely rich and interactive environment. Think Ultima Online, except instead of seeing a graphical depiction of a tree you will read about the tree, have the opportunity to climb it, then subsequently fall out of it and bang your head.
Classes & things you can do in this game
+ Become a sword-wielding witch with ridiculous fighting skills, swoop down on unsuspecting foes with your flying broomstick, before retiring for the evening to brew some healing tea and sell it in a friend's playershop.
+ Become a mugger who steals an opponent's weapon before backstabbing him and then slip into the shadows with a taunt of glee - later you sell the weapon back to him after you've engraved a nasty message on it!
+ Become a questing warrior who can read and write in any foreign language, who can cast his own magical shield from a scroll and who has parry skills so high that his delicate custom-made clothing never takes a hit when in battle.
+ Become a lazy old wizard who spends his time enchanting or portalling people about for a fee, then later kicks his feet up in his private mountain-top cottage. Wandering thieves keep a distance from the priestly wards covering his possessions!
+ Become an untouchable assassin, hunt down contracts worth thousands of dollars in a race against your peers, before grouping with your friends in an isolated desert to keep a low profile while you farm mobs at lightning speed for experience.
+ Become a priest who uses scry to creep up on opponents before magically blinding them and crushing their faces with a two-handed sledgehammer, then later shoplift some expensive spell components as you head back into town.
You will like this game if...
+ you can read (or even if you can't - several blind players use screen readers)
+ you like to EXPLORE (maybe walk off a cliff or get eaten by wolves)
+ you like to quest (proper quests, not MMO fetch quests. stuff that gets your heart pounding)
+ you love customisation (from furniture to tattoos to chainmail miniskirts)
+ you like to discover hundreds of interesting spells and advance your abilities from 0 to infinity
+ you love HARDCORE (jumping out of your seat and sweating your ass off because you got lost and you really really really dont want to die here)
+ you love numberchasing
+ you like split-second decision making ("damn did he go east or west? and what the hell happened to my other dagger? Shit, a bear!!")
+ you love living in an "open world" and unrestricted playkiller environment (becoming PK is optional but permanent once the choice is made)
+ you like detailed systems, intricate interaction and creative writing
+ you cherish FREEDOM in a game, being able to do anything, and having such a rich and humongous world to do it in
+ you are a fan of Discworld. if you didn't know this existed already, then prepare to eat your hat! it is a perfectly precise depiction of the world
Roleplayer, grinder, explorer, handyman, playerkiller or tradesman..
..pick one, or become them all

http://discworld.starturtle.net/lpc/
Click "play now" to open a basic flash client. Make a new character and enter the tutorial zone.
Basic commands (not required reading - just use common sense)
+ Show Spoiler +
l - look around
l <object> - look at object
read <object>
s, e, w, n, ne, up - some movement commands
get <object>
i - display inventory
wear <object>
remove <object>
commands
skills
skills <fighting, magic etc>
qwho - who is online
tell <playername> <text> - talk to someone (eg "tell jimmy hi! how are you?")
say <text> - say something to everyone in the room including NPCs
newbie <text> - talk on the newbie help channel
hold <weapon>
unhold - eg "unhold dagger in right hand"
equip - equips everything equipable
temperature
finger <player name> - display info about someone eg when they logged in
group
wimpy - makes you run one room if you lose too much health
tactics - set your combat attitude
give <object> to <name>
sell
buy
browse <object> - displays info about an item in a shop
kill
bury
sc - display your score
alias <text> <text> - set a shortcut key for something, eg if you type "alias f finger" you can now type "f jenny" to finger jenny.
con <target> - very roughly consider how tough the target mob/player is compared to you
read the help files!
Quick rundown on combat & advancement
+ Show Spoiler +
Kill stuff (you must "bury corpses"afterwards - alias it), use commands and complete quests to gain XP.
Spend XP to increase the level of skills such as fighting.melee.dodge or magic.methods.elemental.air.
Set your STATS so that your skills are given appropriate bonuses (can do this ONCE only). Eg a particular priest might want more int & wis, but another priest might want more dex & con. Dont fuck up (just leave them at their default of 13 for now).
Get yourself useful commands & skills - a warrior might benefit from being able to bandage himself, or he might like to use priestly healing, or he might use 2-handed swords and therefore have high strength and can carry around bottles of healing tea. He certainly could do with being able to fix up his weapon once in a while.
Repeatedly using commands will give you random automatic level-ups (TMs).
You can advance certain guild-related skills in your guild at a reduced XP cost until level 300 (eg some magic skills for wizards, some fighting skills for warriors). After that you need to learn from other players ("teach me x from jenny").
Everything is uncapped but suffers from diminishing gains. That means you can be a priest who is better with daggers than an assassin, or a warrior who is better at shoplifting than a thief.
However there are certain "guild only" commands that only a member of that guild can learn (you cant multiclass or change guilds). Eg only a thief can filch (steal an item that is worn) and only a witch can fly on a broomstick.
TL Community
+ Show Spoiler +
When you leave the tutorial section you can choose your nationality and language. I recommend people "choose morpork ankh-morpork". You'll find yourself in a huge bustling city that is modelled on old London. You can easily find a branch of each of the guilds here and the city is very safe so long as you don't do anything stupid.
Explore. Read the wiki http://discworld.imaginary-realities.com/wiki/Main_Page . Find some quests (quest lists somewhere). Get a map off the web. Join a guild and advance a few levels in fighting or whatever you want (type "cost primaries" in the right room, or you can learn fighting from another player if its not a guild primary).
Form a group with some other newbs. Try not to die - you have 7 lives and can buy a few more but this game is all about not dying. Ask the newbie channel for tips. Most of all relax and enjoy... I enjoyed my first few months just wandering around with no skills!
Type "godmother help" if you get too lost (and are below guild level 50).
Why am I posting this? Idk, i've wanted to for a while. Alongside BW and FF7 this is the best, most detailed game I've ever experienced. Maybe some of you would like to try it