I will give the credit for this idea to Rich Legg as I discovered it on his blog.

I thought I would try it out in GIMP, and it is a pretty easy way to add punch to your photos. Here is my test photo. Now normally I don't do any major doctoring, but I wanted to play around with this photo I snapped a while ago and the van really bugged me, besides taking the van out, here is what I did to this photo:

1. Duplicated Original Color Layer
2. On new layer, made a minor curves adjustment to bring out contrast (Colors>Curves)
3. Duplicated Second Layer then desaturated it to B&W (Colors> Hue-Saturation)
4. In my layers Dialogue, I set the mode from the drop down menu to "Overlay".

That's it! Play around with the curves on the b&w layer or try different layer modes to see how you can tweak it.

Here is the before & after: