Very very good looking opensource RTS, under development for years and probably still a couple of years out of completion. Recently open sourced the entire project and making better progress
http://wildfiregames.com/0ad/large list of tested and reviewed physics based games, free and non free, mostly non free/non open source.
http://www.fun-motion.com/list-of-physics-games/tags: games
PokerTH is a well coded cross platform poker client with network protocol. Written in qt4 with c++
http://www.pokerth.net/Cross platform free open source game based on Colonization and similar to civilization.
http://www.freecol.org/tags: games