I've found a few really, honestly fantastic open source games and I'd like to share them and discover some new ones too.

Firstly, NullpoMino is a fantastic cross-platform Tetris clone. It's highly, highly customizable and is lots of fun to play. It's got a great community over at Hard Drop, and if you like Tetris - you'll like NullpoMino. However, there are a few issues with it; one in particular being the relatively low skill ceiling of Marathon. The skill ceiling is very low (compared to other titles such as the NES edition), but this is solely exclusive to Marathon as far as I know. It turns into a test of patience rather than skill. Another issue is with installation; as the game was made for 32 bit systems it is rather slow, and it's finicky to run on Linux. However, it is a fantastic Tetris title (and open source!)

Another good open-source game is Osu! I've never played it competitively, but it's a fair bit of fun to play with friends and is a great game. It's cross-platform, although I've only ever played it on Windows, and it has a nice community. Fantastic game that I don't see mentioned too much.

The last game for this post is SuperTuxKart. It serves as a great kart-racing game, that's fun and has a low barrier to entry. Graphically, it's not drop-dead gorgeous, but it's not bad at all either. It's decent solo, and a lot of fun with friends. Would highly recommend. Cross-platform and available as a snap.