Since even before I got back into photography in a serious way, I still had a need from time to time to edit images or make graphics from scratch. Being somewhat cheap, I did some looking around to see what the open source and freeware community had come up with as an alternative to Adobe's Photoshop. That's when I found GIMP - The GNU Image Manipulation Program, a program I continue to use.
It is sometimes called the poor person's photoshop, but to me it really is a good alternative. And it's free. You can use layers, manipulate tone curves, saturation, do all the sharpening you want, decompose to RGB and LAB, and the list goes on. Thanks to its open design, there are scads of plugins available, many of which offer pretty advanced features. I often use:
Like many advanced software products (and I guess imaging products in particular), it has a pretty steep learning curve but there are many tutorials and resources out there to get you going.