My How To: Change Icons guide is perhaps one of my most popular posts I’ve ever made. It shows you how to change icons manually. However, lots of people complained about how it was time consuming to change all of your icons, and people were getting pissy about how the only application which does it for you automatically is CandyBar, which you must buy.
While CandyBar is an excellent application, I have bumped into the freeware, almost equivalent version of CandyBar! Allow me to present to you LiteIcon! Almost has all the features CandyBar does. However, one fairly big thing which it lacks is the ability to change application icons (but you can go ahead and do that manually).