I have in all honesty, the bare minimum of kit on this PC - if you exclude Visual Studio 2008 and the DX SDK.
Interwebs
Mozilla Firefox [Web Browser]
Mozilla Thunderbird [E-Mail Client]
uTorrent [Torrent Client]
Windows Live Messenger & Messenger Plus! Live, of course
Media
ImgBurn [For burning ISOs and also just making CDs/DVDs]
iTunes [Cause I have an iPod
]
K-Lite Codec Pack [For watching movies]
QuickTime [It's installed with iTunes]
Office
Mozilla Sunbird [Calendar]
FoxIt PDF Reader [It's less bloated than Adobe Reader, but has No Fx (Firefox) Integration]
OpenOffice.org 2
System
CCleaner [De-junker]
You could probably get away without Sunbird if you're on Vista. Also, I prefer OpenOffice to Microsoft Office, no particular reason though. [Except how much disk space Office 2007 uses!]