http://youtu.be/R8_tL7UCXuc
This effect is called HDR:
High Dynamic Range Imaging.
Basically, it makes the image look much more colorful and makes it “pop”. You can play around with the settings depending on the image.
This tutorial was made by jneurock on Gimpology in text form, I just couldn’t NOT tell you guys about it though, such a useful technique.
Here’s the URL:
http://gimpology.com/submission/view/fake_hdr_look_in_gimp/