http://www.moddb.com/engines/xash3d-engine
Sorry for the link to moddb, but it seems to be the only one of which I can find the newest version of the engine, the newest "Xash3D Engine v0.95" appears to contain source code.

The Half-Life engine had a software renderer and in theory this should too, the problem remains that I do not see a linux port anywhere, which could obviously make this a little harder to port.
It appears to be in C++.

Apparently this engine runs most mods as well as half-life, so I'm imagining counter-strike would be possible, albeit without multiplayer.

A port would be appreciated if possible.
Thanks
ShadowZero