Photofiltre, Paint.NET and GIMP are the best free ones imo. Paint.NET was wanted to be the replacement for the crappy MS Paint, while GIMP is a simpler version of Photoshop. Don't know much about Photofiltre, but you get the idea. It depends what you are more comfortable with.
I have Photoshop, but for Logo/Banner purposes, I find Paint.Net is easier to use and has exactly what I need. I use Photoshop for more complex projects though