Yesterday I visited a neighbor who uses Windows XP on her computer to help her with a problem she was having and I noticed how each folder on disk D: had a cheesy little icon on it to personalize them a bit.
Do this with Nautilus (the File Manager of Gnome) it's really simple and we don't have to use any extra software or anything like that. All we have to do is:
1- We select the folder we want to customize »Right click and click on the folder icon.
2- We look for the icon inside our folder .icons, or we simply select any file in format . Png, .jpg, .svg (others can be used). Here you can find some very cool ones. And ready!!!
This is what mine look like 
