Candybar is free and I think it works for Mavericks. Check before you download it. I can't remember if they stopped support with ML or Mavericks. You can also do it manually I'm pretty sure I wrote a guide on it so check the guides section.
I've been doing it in finder for as long as I can remember:
Click the folder/app you want to change the icon for, and hit command I.
Do the same with the icon file you want to use as the icon for this app. Click the icon in the top left hand corner and hit Command C.
Click the same icon in the folder you want to assign the icon to, and hit Command V.