One of the spriters from the Terraria team decided to start up on a new project earlier this year called Starbound. It's basically Terraria with ever so slightly better graphics and a much more expansive universe (literally).
The game is still currently in development, but when I checked out their website, I was very excited for what they're doing over there. You will be able to play on multiple planets with their own generated biomes and landscapes. You will be able to explore, collect fauna, craft items and mounts(!), and customize endlessly. I have been on their site less than an hour but this game has gotten me very excited!
If you guys want to support Starbound's development, you can pre-order here.
The game is currently in development, so I will update this post as more info is released! Update (4/28/2013): Added Glitch, Roadmap, music, and pre-order links.
Keeping a close eye on this one. I've put 200+ hours into Terraria and this looks really promising, even though it might be quite different gameplay wise.
I really didn't like the idea of Terraria originally, but I was playing Minecraft at the time and had only ever heard it described as a 2D Minecraft, which didn't sound appealing at all. That description doesn't come close to covering the depth that the game offers and boy was I glad I gave it a try anyway.
Now Starbound is a thing, I can't wait to get my hands on it. It looks like it will really expand on the strengths of the first game (exploration, procedural content generation, tech progression). Definitely one to watch.