Lomo-Gimp tries to give your digital pictures the warmth of those taken by an old rangefinder analog camera. Think of it as a Kodak Instamatic or a Lomo Holga faking effect. It enhances contrast and saturation, and applies a light and a dark overlay, trying to emulate incorrectness in exposition. Please note this does nothing to emulate the chromatic aberration peculiar in pictures taken using a real Lomo.
Get it at Lomo-Gimp.