I love being able to play those old games on a modern computer. A lot of these have enhancements to the original. And being able to see the source code that makes them tick isn't too bad either.
Misc. source ports
Ports of Heretic, Hexen, Wolf3d, Descent, Duke3d
Dave Gnukem, a Duke Nuken 1 port
Duke Nukem 3d
Quake source code
KDX, a DOOM engine front end
DOOM WAD station
The DUMB engine
DOS DOOM, the original open source DOS client ported from the linux source!