FreeCS is a free-software reimplementation of Counter-Strike 1.5.
...Just download, play, contribute.
The game-logic is written by me with help of twhl.info's wiki.
It has since been incorporated into the Nuclide project, which re-implements more than just CS 1.5.
SDL.NET is a set of object-oriented .NET bindings for the SDL gaming library and provides high-level access to audio, keyboard, mouse, joystick, TTF fonts, various image formats, sound mixing, MPEG-1, and 2D video framebuffer.