I have a number of folders that I put down on my dock. Applications, utilities, downloads, my programming software, my photography software, games, and so on. The generic blue folder icons all look pretty much the same, so I thought I'd liven them up and make them easier to distinguish. A clone trooper for the games. A camera lens for the photography stuff. Etc.
So how did I do that? Here's how: First, I found some icons I liked. There are tons of nifty icons at InterfaceLIFT. Download the ones you like; put them somewhere accessible. Second, find the folder whose icon you want to replace. For example, use finder to navigate to your downloads folder. Third, get info on that folder (use command-i or select "get info" from the file menu of the finder). Move the info window to one side. Fourth, find the icon you want to use. Get info on the icon file. Fifth, with the two info windows side-by-side, select the icon you want to copy (the icon in the upper left corner), copy it, and then in folder window select the icon you want to replace, and paste. The generic blue icon should be replaced by the snazzy new one. Spiffy.
Let me know if that works for you.