http://playstarbound.com/
They have recently set up a legitimate office and let people play their nightly updates. They're now getting updates out more frequently as the developers are all moving to their office location.
Why is it awesome?
Modding
I've started 2 of my own mods which I haven't worked on much, here, and here.
The modding community already has some amazing things available. During my mod spree a while ago, I've discovered the sort of potential we've seen in the mapping community in the past (i.e. limitations seen as "impossible" can often be done through workarounds or through ideas people have never tried before). It's a very attractive environment.
A great mod idea I've recently had is the possibility of Garry's Mod-style objects, by connecting things together and getting them to move. This would possibly use "invincible friendly monsters" that are fully scripted and made to look like random objects (I've used this technique, it works very well), and a technology for interacting with them.
Updates
You get all of the update information and can even play with nightly builds. The game is good right now but there's still a lot more stuff to come.
Multiplayer
Like Terraria, the game is multiplayer. However, you get more than one world to work with in the game's universe. It would be cool if we could have some sort of SEN solar system where everyone gets their own planet.
NPCs
You can spawn your own NPCs and as many as you want. You can discover NPC towns. You can script your own NPCs to do cool things (see mods). NPCs are capable of doing a lot if scripted correctly.
