There are several, some a lot more expensive than others. Photoshop (expensive) seems to be the most popular. There's a "stripped-down" version, Photoshop Elements, and also Paint Shop Pro, which are about the same price. There are also some free programs, like The Gimp, which is said to be very complicated, and Photofiltre. If you browse through this forum, or do a search using the names, you'll find info and links.
This thread might help:http://www.thecatsite.com/forums/showthread.php?t=76789