18 GIMP
Adobe Photoshop is the industry standard when it comes to image manipulation. It’s also pretty expensive. If you’re looking for an alternative that fits into the $0 price range, then GIMP is a pretty good contender.
GIMP is a completely free cross-platform “image manipulation program.” It packs a range of powerful features and tools that you’d be hard-pressed to find for free. While there the interface isn’t perfect, it’s pretty modular and customizable. There’s also a bit of a learning curve, but there’s one for Photoshop, too.