I love the guys at Iconfactory, and their CandyBar software is the best way to switch icons on your Mac. They also have some amazing collections of icons as well, so check them out. If, however, you’re looking for a free alternative to CandyBar, you may want to check out LiteIcon.
LiteIcon is freeware, and is a slick and easy way to change out your system icons. The main site isn’t in english, but it’s pretty easy to understand, and the application has support for a variety of languages.
Check it out here.