Good, free software?
Oh, there's so much I use everyday.
Pidgin for instant messaging,
kvirc for IRC
mumble for VoIP with my clan,
7zip for zipping,
gimp for image editing,
foobar as audio player (very lightweight, alternatives would be soundbird and winamp),
firefox as browser with some nice addons,
thunderbird for email,
sunbird for calendar,
linux as the better OS, love it for playing around,
hlsw for game server browsing,
utorrent for torrents,
emule for emule network,
blender for modelling,
virtualbox as a superb virtualisation box,
filezilla for ftp,
openoffice for all the office writing and maths,
winscp and putty for remote administration,
notepad++ for programming and text editing,
virtualdub or virtualdubmod for video encoding,
audacity for audio editing,
gnuplot for plotting,
defraggler for defragmenting,
and more... :D