This is a free, open source photo editing software, similar to Photoshop. Its completely free, and the best part is that there are tools, built by users, to do almost everything that Photoshop can do for a casual user. Of course for professionals, Photoshop is the best alternative in my opinion, but for free, nothing beats, and I mean NOTHING beats Paint.NET
Paint.Net and the Gimp are both great, Gimp can't do vector graphics though and I'm not sure about Paint.Net.
As for Photoshop: <3 photoshop, </3 price