Sometimes, I take screenshots, and I like to circle relevant bits of info. The screenshot is easy, using ksnapshot, save it as a PNG file, then open it in Gimp. That's where the fun starts - how do you make a circle (or ellipse) in Gimp?
The answer is: make a new layer, select that layer, on it make an elliptical selection, then fill the ellipse with whatever colour you want to make the line. Then shrink the selection (by, say, 3px) then cut, to remove the centre of the selection. Hey pesto! A neat elliptical line.