Operating System
Vista x64 - Well, it works fine for me and 32bit system is not really an option due to memory limitations.
Internet and Security
Opera - Daily browser for me
Chrome - Browser for iRacing, works faster than FF. (security settings in Opera do not allow browser to access system services)
Avast! Antivirus - Because it's free and has autoupdates that do not require your attention.
Comodo Firewall - Basic version is free and it's much more configurable than other alternatives like ZA.
Spybot S&D - Removes adware, although I haven't yet seen it find anything else than tracking cookies.
Multimedia
Media Player Classic - Very lightweight player and works well with FFDShow
VLC Media Player - Just as a backup, used for flash video but those seem to work fine now on MPC too.
FFDShow - Codecs and filters you need for high quality playback and encoding. No ads or any unnecessary included like in some codecpacks.
VirtualDub - Very powerful lightweight video editing software, I use it mainly for cropping, resizing and encoding raw footage into smaller formats.
Winamp - Been using it for years now, I don't really care for some of the latest features so I leave them off.
Exact Audio Copy - Best sofware in my opinion for encoding your audio CD's to MP3 format.
Other
Open Office - I don't use office tools that much to make Microsoft package worth purchase, but this one is good alternative because it's free.
Fraps - For recording ingame video.