Google Apps - free for nonprofits, can customize, use your own domain (use the address
someone@somesite.com with the Gmail interface, features, etc.), and such.
A couple of others:
XAMPP - invaluable for local testing of websites, themes, etc.
Free Download Manager - Downloads large files fast, with no continuity problemt(you can pause, restart from an unexpected shutdown, etc. unlike, say, the Firefox download manager). This works great for downloading things like 3DS Max or Inventor (faster and more effectively than the FIRSTBase downloader program from Autodesk)
EDIT: Just thought of this one (because I need to use it):
nLite. It allows you to customize Windows XP installations (a similar tool,
vLite, is available for Vista). It lets you slipstream updates (service packs, as well as normal updates), drivers, settings, etc. into the installation. You can preconfigure users, product keys, themes, network settings, etc. You can also tweak a few things, or remove unneeded things (that annoying dog in the search menu).