FIGS, open and fun! GPL/java game server for many games, and a graphical client. Server works for 16/19 chess variants,adding go, rating system, and haskell AI client soon.
Signus: the Artefact Wars is a highly-playable turn-based strategy game with 19 long missions, lots of precisely designed units and very good AI.
Maintained fork is at https://github.com/signus-game/signus