http://xtux.sf.net
an X-based game similar as CS2D, including single player and multiplayer game
you can choose your favorite free software or "open source" character (including GNU, Linux Penguin, FreeBSD, OpenBSD, KDE, GNOME, RedHat, SuSE, Richard Stallman, Linus Torvalds, etc.) and start fragging
as this only uses X and not KDE/GNOME specific, you can run this game under most of UNIX-style systems, including GNU/Linux, *BSD, and even the incompleted "real" GNU system (with Mach/Hurd as the kernel). This should work with Cygwin with X Window system if you are using Windows
http://www.cygwin.com