Windows:

• Source Insight – still the best editor for C/C++ code
• cygwin with mintty (run it as C:\cygwin\bin\mintty.exe bash --login -i)
• firefox and chrome
• 7-zip
• WinMerge
• Notepad2 and/or Notepad++
• WireShark
• Consolas font (if on Windows XP)
• Python (ActivePython build)
• Subversion (SilkSVN builds)
• SysInternals live, regular
• TaskSwitchXP (XP only, not working and largely not needed under Vista and 7)
• digsby for IM client
• NSIS
• Virtual CloneDrive for mounting ISO images
• Media Player Classic – HomeCinema (an update to Media Player Classic)
• IcoFX – free icon editor
• Greenshot – free screenshot grabber
• WinDBG

Visual Studio add-ins:

• RockScroll
• SonicFileFinder – a better way to switch between files, assigned to Ctrl-O to match Source Insight

Firefox extensions:

• Foxmarks
• ietab
• AdBlock plus

Mac:

• MacPorts (MacPorts essentials)

Misc Windows:

• another list of useful software
• nirsoft.net – CurrPorts, SmartSniff, OpenedFilesView, WhatInStartup etc.
• FileZilla