The tutorials at http://www.gimp.org/tutorials/ are very useful. You may come to astonishing results following them. My favourite ones are
- http://www.gimp.org/tutorials/Color2BW/ (How to create good grayscale images from coloured ones)
- http://www.gimp.org/tutorials/Film_Grain/ (How to add film grain to make the image look somewhat vintage)
- http://www.gimpguru.org/Tutorials/FilmGrain/ (dito)
Another nice tutorial that deals with creating vintage look using The Gimp is here:
It focuses on more subtle filtering.