I have tried almost all of the Photoshop alternatives and most are disappointing because either they are too basic, the interface is unappealing, it doesn't work with enough image formats, or any other of a zillion reasons they're just not Photoshop.

Of all the alternatives, the ones I've used the most are GIMP and Artweaver. GIMP was my alternative of choice for years. I created entire arsenals of website graphics using GIMP (and never once forgot that it wasn't Photoshop). It was adequate.

I don't remember how I first became aware of Artweaver, but the look and feel is so far ahead of GIMP, it's like comparing a Mercedes CL600 and a Honda Civic; they'll both take you where you need to go, but I'm pretty sure you'll be a lot more comfortable with the Mercedes.

Artweaver shares the look and feel of Photoshop; those who are familiar with Photoshop won't have much trouble acclimating to Artweaver. It doesn't have the breadth and depth that Photoshop has with respect to filters, etc. but for the basic user who needs to do some general image editing it'll be more than equal to the task.

I literally use Artweaver every day at least a few times. It has become an indispensable tool in my daily workshed and I'm thankful that I have it. I encourage everyone to try it out and when they love it like I do, I remind them to tell others about it. If we can build a robust community of users, maybe downloadable goodies like filters will be more widely-available.