PULSAR: Lost Colony is an excellent indie title where up to 5 players will command a spaceship to explore the galaxy. Although it's still WIP, it plays incredibly well and is very worth picking up.

Although it may no perform well under big amounts of pressure, it's one hilarious game. It's also better with mods, of which there are a decent selction already.

Though clearly still a work-in-progress, this little arcade shooter is quite a lot of fun, especially with others. Either fight AI/Your Friends in Aracde Mode or take on an endless campaign in coop.

Space is the Final Frontier. Words that could not be more true in this indie gem. Construct and command spaceships into space (hopefully!), or build a spaceplane and deploy it around Earthlike Kerbin.

The next generation of Theme-Park Simulations is Spectacular. Although the management is a tad shallow and the performance is bad in bigger parks, it's creation tools are a revolution for the genre.