Today I let the videos speak:
First of all a nice FOSS bullet hell SHMUP, called Shmupacabra:
Taisei Project is maybe also relevant if you like getting killed ALOT (hehe, insider joke for q).
You can support the developer of this nice puzzle game by buying the Expansion-pack for 2 Canadian bucks or more! Another option to try and fund FOSS game development… maybe there will be some sales figures published at some point?
Next on the list: open-source engine reimplementation project Corsix-th:
You will need the original game for the non-free artworks though.
And last but not least: 0 A.D. got a brand new website: www.play0ad.com! Celebrating this they also made some nice video tutorials for total beginners: