One of the really cool things about OS X is that there are a lot of freeware and shareware programs out there. A lot of them are just small, "one trick pony" applications that only do one thing, but they do it really well. Here are a few of my current useful favorites and a couple of links to find your own.
VersionTracker is the ultimate download site for programs.
Apple also has a MAC OS X Hot Apps Site.
If you want to get started with open source software check out SourceForge or SchoolForge for education specific open source software.
iSkysoft, The Free Mac, and The Free Mac Classroom are also great resources.
Widgets all over the place - get your Widgets here!
Jiggler - wiggles your mouse periodically so you computer doesn't go to sleep and the screen saver doesn't come on.
Want to spotlight an area on your screen? Try Mousepose.
Need to get your music off an ipod and back into your machine? Then you need Senuti.
Pencil is a great freehand drawing and animation tool.