Sidebar Styler
http://www.stoyanoff.info/blog/code/styler/
Stardocks Iconpackager which is good for easily changing icon sets.
http://www.stardock.com/products/iconpackager/
CursorFX can be used to change mouse cursors
http://www.stardock.com/products/cursorfx/
Rocketdock (free) or Objectdock (not free?) is a good Mac-like toolbar
http://rocketdock.com/
And if your feeling adventurous search for Vista Themes which will lead you to sites that have user created themes which replace Aero and do not use additional resources. Installing themes requires patching a few files, but it can be worth it.
Deviantart.com has some good stuff.