What is Gimp?

GIMP is the GNU Image Manipulation Program.

It's a free (open source) app for image editing and stuff. Many people claim that it's better than Photoshop, or at least as good as Photoshop.

Now, you won't hear something like that from me, but I'll tell you that it's quite good.
It has most of the stuff you can find in Photoshop though. And it has scripting capabilities.
It offers batch processing, effects, image render and other stuff.
If you're an amateur user it could substitue Photoshop.
If you're a pro it could still help you out, and maybe save you some money needed for a Photoshop license (for example, you could buy Photoshop for one PC and use GIMP on others).
It opens lots of formats, has support for Photoshop files (PSD), and scripts for it can be written in Scheme, Python and Perl.

And it runs on Windows, Linux (UNIX) and Mac OS

I don't use it exclusively, but I use it a lot.

I'd recommend you to try it.

GIMP - The GNU Image Manipulation Program

If anyone has some opinions or uses it, I'd like to hear it.