The gimp, but to be honest, if you are looking for a professional look, you definitely can't beat photoshop... I prefer illustrator actually, because of the ease to create scalable graphics which do not loose sharpness as you scale them up.
Also the typography and anti-aliasing of the adobe products cannot be beat. It might be worth just downloading a demo to be able to work with.
I'd also recommend Gimp as a free alternative, but it's not the most intuitive editor out there. It does involve a bit of a learning curve, but it's worth taking the time to learn as it is extremely powerful once you know your way around it.
If you're running Windows, Paint.NET is another free image editor. IMHO, it's one of the best free image editors for Windows. I've used it for years on my Windows machines at work. Definitely recommended!
Paint.NET is powerful, high quality and is intuitive and easy to use, so it's great for editing images quickly and easily. It supports a wide range of file formats and there are a lot of built in image effects. Plus it's extensible via plugins, with lots of additonal plugins available on the paint.net forums.
The only potential downside is it requires the '.NET 3.5 sp1' runtime to be installed. But if you're running Vista or 7, that shouldn't be a problem as .NET should already be installed. However if you're running XP, you'll need to download and install the .NET runtime if you don't already have it! (Saying that, I think recent Paint.NET installers will automatically download and install the .NET runtime if it isn't already present!)
GIMP is certainly the best free general purpose photo editor.
For some it is at first uncomfortable to use with all the different windows but gets easier as you get used to it. And there are plenty of tutorials on youtube of how to use the different features.