Remove unwanted color casts from your photos.
Make colors brighter and contrast sharper by using color curves.
Create 3D looking text with a mirrored ground reflection.
Make parts of your photos color, and other parts gray.
If you erase and see only white color, then you must add an alpha channel to your gray layer.
Click Layer > Transparency > Add Alpha Channel
More Rare things to do with Paths.
Text Along Path
Selection to Path
GIMP 2.8 UPDATE:
For text along path, right-click on the text.
Learn how to use the most common functions of the paths tool.
Create a Yin Yang logo using guides to properly place your circles.
Use layers and the Path Tool to create some nice word art.
The best tutorial on making realistic looking tattoos on skin (or any surface). This works with black/white and color tattoo's.
Keep foreground objects in focused and clear while making the background blurry.