I actually have a list (Score 1)

Considering the frequency with which I reinstall, here's my list for Windows (post-driver updates):

Mozilla Firefox - a must. even if it's not a necessity straight off as a browser, popup blocking makes it worth having immediately.
Trillian - what friends don't enjoy the logging on and off every 2 minutes as you have to reboot your comp for new settings to take place
SmartFTP - as a web developer this is a must, can't pretend to work without an FTP client.
Winamp (5) - I use it for everything media now. I'm an addict.
WinRAR - I know it's shareware but I still like it's ease of use and modifications to the Windows context menus.
Nero - my burning software of choice
ConTEXT - my editor of choice; see SmartFTP
Google Toolbar - unfortunately, I end up doing a lot of testing in IE and without this, I might as well kiss my peaceful browsing goodbye.
Spybot Search & Destroy - not so much of a necessity immediately, but the immunization qualities are great to set up from the get-go
Ad Aware - see Spybot