I cannot believe nobody mentioned these programs before:
ClamWin, the best free anti-virus solution...oh yeah, it's Open Source too:
http://www.clamwin.com/
µTorrent, the best BitTorrent program since sliced bread:
http://www.utorrent.com/
Free Download Manager, like the name says, speed-up your downloads with this program (yes, even broadband):
http://www.freedownloadmanager.org/
CPU-Z, for identifying what's running under the hood of your PC:
http://www.cpuid.com/
Foxit Reader, the only PDF Reader you'll ever need, honestly:
http://www.foxitsoftware.com/
Gadwin PrintScreen, grabs screenshots automagically when pressing "PrintScreen":
http://www.gadwin.com/
Gizmo, you can say goodbye now to proprietary Skype:
http://www.gizmoproject.com/
KoolPlaya, the All-In-One Media Player:
http://www.koolplaya.de/
Notepad 2, M$' original Notepad-killer:
http://www.flos-freeware.ch/
Restoration, the ultimate file undelete utility:
http://www.softpedia.com/get/System/...toration.shtml
TCP Optimizer, for tweaking every last Bit from your connection:
http://www.speedguide.net/downloads.php
Windows Worms Doors Cleaner, making sure that security holes are indeed closed down:
http://www.firewallleaktester.com/wwdc.htm
M$ Cleartype Tuner, tweak your display settings to hell and back:
http://www.microsoft.com/typography/...ner/Step1.aspx
And of course, last but not least...BOINC. Help Humanity by using your CPU's idle processing power to find a cure for deadly diseases, among other things:
http://boinc.berkeley.edu/