Is there any good picture editing programs that I can use?

I love to take pictures but also edit them. Now i'm using Picasa 3 and Microsoft Digital Image Pro. Is there any good picture editing programs that aren't that expensive(im only 13 and my parents don't really believe or support me when i say i love photography so i don't have that much cash) or are free for you to download? thanks if you know, i would really appreciate it.