Steam is having a 4th of July sale starting now. Save money on Wolf 3D, Spear of Destiny’s 3 episodes, RTCW, Wolf Pack, or get all of The ID Super Pack for $29.99. The ID Super Pack apparently includes all of The ID Software games made before 2007 except for ROTT, Strife, and HacX. HacX is now a freeware Doom 2 add-on here.
The RTCW Game of The Year map pack isn't on Steam, but it's free here.