Chocolate Doom
Conservative Doom source port.
Freedoom
Crowdsourced Doom IWAD file.
Irmo
Client-server multiplayer gaming engine.
WADptr
Doom WAD file compressor.
SMMU
Older Doom source port.
Lhasa
Free LHA implementation.
C Algorithms
Liberally-licensed C algorithms library.
Yoctolisp
Lisp interpreter written in a weekend.
Agito
Subversion to Git conversion tool.
SDL Sopwith
Port of the classic game Sopwith.

Hackery

Doom Apocrypha
Various things I've written about Doom.
Sinclair QL disks
Reverse engineering some old floppy disks.
Password teacher
Program passwords into your muscle memory.
Automatic SLIGE megawad
A new 32-map Doom MegaWAD every day.
BSD License generator
Generate a license for your project.
Brainfuck.NET
Compiler for the Brainfuck programming language.
Self-modifying Brainfuck
Brainfuck language variant.
Tools...
Various tools and small programs.