Let's say I've accidentally shot my pictures on "sunny" white balace when I should have used "shade". What is the easiest way to warm the pictures up a bit in Gimp? The "Colors/Color balance" tool does not really cut it, as I don't want to mess with three sliders for three brightness levels.
The quickest GIMP tool I'm aware of is the color levels tool. Near the bottom of the tool is 4 buttons: Auto, black point, gray point, and white point. You can use the auto (iffy, but it may get it right or close for minor adjustment) or use the white point button to select the white point in the image.
It is, by the way, the same technique I suggested for Photoshop in the first link @mattdm supplied.