I use Windows but I ordered a Mac (I'll get it in the next 10 days). I need a bigger monitor and I need a Mac so I combined both requirements into one 24" iMac :)
My favorite peaces of software are Total Commander, Firefox, Zend Studio and Trillian. They make my life whole lot easier.
I'll answer the fav software question too:
Quicksilver - my Mac wouldn't be the same without it
Safari
skEdit - code editing program
cssEdit - just what it says
SoHo Notes - a notekeeping app
OmniOutliner pro and kgtd
Transmit - ftp program
Firefox
Thunderbird
CyberDuck - FTP
Smultron - code & text editor
NeoOffice - OpenOffice on Mac
GimpShop - The Gimp with a Ph*t*shop like interface
Pages - the "stylish word processor" from Apple
MAMP - Macintosh, Apache, MySQL and PHP
Ilija Studen - What OS do you use to develop activeCollab? and what's your favourite software?
Off-topic? That's why it's in the Chat section!