Like everybody else has mentioned, Photoshop is the defacto standard. For the money though, Paint Shop Pro is hard to beat. The only Adobe product that comes close in it's price range is Photoshop Elements. I've been using PSP since version 5, and for me it does everything I need. PSP-X is good, but it's got a few quirks. [img]/ubbthreads/images/graemlins/smile.gif[/img]
Think of Paint Shop Pro X as a cross between Photoshop and Photoshop Elements. It can't do EVERYTHING that Phtoshop does, but it has a few more features than Elements.
Alternatively, if you have a Wacom tablet (I've got an older Graphire tablet, but any model qualifies), and it came bundled with a copy of Photoshop Elements, you qualify for a $300 discount off of the full retail version of Photoshop CS2. Click on the link below and fill out a 21 question survey, and you'll be redirected to Adobe's site, and the discount will automatically be applied. You'll have to enter in your PSE serial number (AT ADOBE'S SITE).