This is going to be a realy list, going in order of operating system I use most. (I won't list the old os's I use like MacOS 8&9)
Mac OS X
Quicksilver - Gets stuff done faster, quicker ways to do things.
Bean - Word processer
iWorks - Like MS office except from Apple.
Neverball - Game thats good for free time.
Transmission - bittorrent client
Cyber Duck - FTP program.
Taco HTML edit - HTML editer.
Adium - IMing.
Altitude - awsome game I play.
Pixelmator - Image editing/like photoshop.
Sketchup - 3D design software.
CrossOver - Runs windows crap on a Mac.
Windowmaker - Window Manigment/GUI.
Kmail - email app.
gl-117 - game.
Openoffice - MS office except free and for linux.
Kopete - IMing.
Opera - web browsing.
GIMP - Free image editor.
Windows 98* to XP
Firefox/Opera - Web browsing.
Thenderbird - Emaill program.
AVG free - Anti-virus.
a-squared - Anti-crap.
OpenOffice- Like MS office but free.
VLC media player*Yes I still use Windows 98 once in a while