• Google Chrome (Web browser)
• Thunderbird (Email)
• Firefox (Web browser)
• Irfanview (Image viewer)
• Pidgin (Multi-protocol instant messaging)
• Winamp (MP3 player)
• iTunes (MP3 player/organizer)
• Exact Audio Copy (CD copier)
• Vim (Text editor, not for the faint of heart)
• Gimp (Image editor)
• Audacity (Audio editor)
• FoxIt (PDF viewer)
• 7-Zip (Archive manager; UI is flaky but handles damn near everything)
• AVG (Antivirus)
• OpenOffice (Office suite with MS-Office compatibility)
• Perl (Scripting language)
• Putty, WinSCP, and TightVNC (Remote access)

These are among the first programs I install on any new Windows install, and I find myself completely flailing without them.