FreeCS
FreeCS is a free-software reimplementation of Counter-Strike 1.5.
FreeCS is a free-software reimplementation of Counter-Strike 1.5 running on the FTEQW engine.
All it is, is a game-logic re-implementation in QuakeC, an architecture independent programming language. FreeCS is not a game-engine as it relies on FTEQW (released under the GPL V2).
All this makes it possible to play FreeCS on virtually any platform! No DRM!