Of course Adobe Photoshop CS is the Gold Standard. It also commands a VERY high price.
Here's a couple of alternatives:
Adobe Photoshop Elements, about $100 though it can be found for less, Basically a watered down version of Photoshop and somewhat more user friendly.
Paint Shop Pro, Very nice program that lists out around $130 (again, can be found for less). Has most of the features of Photoshop and is priced much better. It also has support for Photoshop Plug In filters.
GIMP (GNu Image Manipulation Program). Best as it's free. Built originally for LINUX but has been ported over to Windows and MAC.
Those would be my recommendations. I use Photoshop 7.0 and though there is a steep learning curve (no way have I scratched the surface of what can be done) it IS very good. There's a reason why Adobe is considered the "standard". I've used Elements in a pich and for the folks just starting out, it's a pretty fair option. I've also used older versions of Paint Shop Pro and really did like it. However, it wasn't up to par with Photoshop and I eventually invested in that.
I've heard good things about GIMP, but I've never used it. Looking at the web site and the various things out there about, it looks to be adecent image editing program. I might have to download and try it.
Hope that helps!
John Diel