ShapeShifter is a very cool "GUI tweaker" that enables you to change virtually everything about the appearance of OS X with various themes (of which there are a great many to choose from). Version 2.4 adds support for windows with a "Unified" toolbar, and includes several bug fixes and speed improvements. ShapeShifter now requires Mac OS X 10.4 or later.
Mighty Mouse is for customizing your cursor - all your cursors - on the fly and with ease. The interface is drag & drop and copy & paste. Doesn't get much easier than that. You can add animations, change cursor sizes and even create your own custom cursors. Version 1.3 requires Mac OS X 10.3 or later and includes a more streamlined interface and a handful of bug fixes.
More detailed info about each can be found on Unsanity's blog.