Browsers
Firefox
Opera
Media players
WMP
Win-amp
MPC
DOOM editors
DOOM Builder
Wintex
Dehacked
DMAPEDIT
Quake II editors
Qoole
Worldcraft
GTKraidiant
IM programs
Trillian, Dont even debate about this!
GAIM
YIM
mIRC
DOOM source ports
SkullTag
ZDOOM
ZDameon
Risen3d
BOOM
Games
Quake II
Quake shareware
DOOM
DOOM II
DOOM3
Driver
Rollercoaster Tycoon
Dangerous Dave
FS2K4 ACOF
And too much more...
Misc.
Macromedia Flash
Game maker
DOSbox
Graphics
M$paint
Paint
Photoshop
PAINT.NET
GIMP
Sound and music
winDAT
Orch
Anvil Studio
Nero wave editor
Theres lots more, but im too lazy to post them.