CCleaner - Handy application to optimize Windows, cleaning out temporary internet files, internet history, registry, and other things, thus freeing up more disk space as well!

Mozilla Firefox - Free web browser. Lots of add-ons and extensions and cool themes! Also a portable version that can be run off of a flash drive @ school etc. available here

Maxmem - RAM defregmenter/cleaner that will free up RAM when necessary. Results in smoother computer operation, better gaming, etc.

RealVNC - Free remote desktop server & client utility. Access your computer(s) from anywhere.

ZoneAlarm - Free firewall for your computer. Many options, works well for me. Stealth mode etc.