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GIMP: GNU Image Manipulation Program
I liked.
Er, I was told it's the free less complicated albeit less effective sibling of photoshop.
I still don't know what to say about that because I am quite a dork when it comes to photoshop, I like GIMP though...good fun.
Only defect I could think of was the tad complicated installation process (please note, here I am assuming that you are an absolute idiot), it requires the installation of GTK+ Runtime Environment before you can install the GIMP from the installer.
Sigh, fifteen minutes of boredom and I create a rather strange picture of Jimmy Page..good fun though.. :D