Vignetting is where the light distribution isn't even from edges to center.
Interesting. Partly because there are software filters that people use to actively get that effect, but also partly, because I figure with software it'd be less of an issue, because people are able to take a wider shot than needed and then crop it to their desire on the computer (at least, that's what I do).
Speaking of software, I've heard good things about Krita but that's more for drawing than for editing photos. The only other open source program I know of other than GIMP is LightZone which strikes me as a simplified version of Adobe Lightroom. Wikipedia has a whole list of programs, so I'm gonna do some looking into that this week as well.
GIMP? I'm pretty decent at it, but I dunno, I'm stubborn in the "making things work whether or not it's easier to get something new" kind of sense. But then again, I never thought about taking photos that I really care to edit really well and now here I am.