I'm a die hard Adobe user, been using their programs since the mid 1990's, so I can't really bring myself to switch to any other substitutions. In the late 90's-I dabbled a bit with Paint Shop Pro and Photo Impact, but once I discovered that I could get Photoshop dirt cheap through my student book store on campus, I just stayed with Adobe products.
I did really like Pixlr, though, after playing around with it. It's got a layout very similar to Photoshop, and would be a great option for people without Photoshop. Don't be fooled by it being a browser based app, it is really powerful and full featured.