It is a real-time, multi-player space warfare game. Both curses and OpenGL clients are provided. It looks a bit like 2D Star Trek ship battles (not stereotyping there, it just does).
So, want to be brainier? Well, Brain Workshop might just be the answer.
Brain Workshop is a free open-source Python implementation of the Dual N-Back mental exercise. This exercise is the only mental activity that is scientifically proven to improve your short-term memory and fluid intelligence (IQ). The game involves remembering a sequence of spoken letters and a sequence of positions of a square at the same time.
Remember Portalized? I kicked up a bit of a fuss over it several months ago. Well, it has it's own website and forums now, and seems to be coming on a bit. It's not yet open source :'( but a bit of positive encouragement might make that happen.
There's a new stable release for VDrift. I'm not sure that it's much different in terms of features to the last VDrift release as lately the development effort has been on a refactoring / rewrite of the code (which is progressing well).
Also Simutrans 100.0 aka 1.0 got released. This really snuck under the radar as it's a very good open source game and not very well known in comparison to OpenTTD, but it's totally Free Software - something the OpenTTD guys are trying to fix but have a lot more work to do on.
The Simutrans team lost their previous forum when switching from one host to another. Which means the Simutrans website is a bit broken (most information was kept in the forums, so many of the links now just don't work). The main issue with the stable 1.0 release is that there is no official release of pak128 (the hi-res graphics pack) for it, although you can try your luck with the nightly builds which does have a section for the latest version of pak128. I'll leave you with a video of Simutrans pak128 although watch out for the dodgy music: