Flyakite is a transformation pack that replaces images in system files. It changes all the icons to default Mac OS X icons, as well as changing the overall look of the system.
If you want to change only some icons you can use:
Icon Packager http://www.stardock.com
Bee Icons http://www.beeicons.com/
(this is something different from Stardock's IconX)
You can also try using BricoPacks that essentially do the same thing flyakite does, but on a lesser scale. You can get some info on them in this Aqua-Soft thread