My favorite image editing effect is solarization. People don't use it very often, because for the vast majority of photos, it isn't very impressive. However, with the right image, the effect can be spectacular. It works best with backlit images, where the foreground image is too dark to be clear. Solarizing lightens up the darker parts of the image without giving them a washed out-look, and it takes the lighter elements and makes them into a color negative. You can read all about solarization here.
I've put together a page with some of my better solarized images. (For technical reasons I couldn't use the mouseover effect directly in the blog without monkeying with the template.) To see the effect, just mouse over the before images. To go there, click here.