I second SAI, but if you want something really basic you can look for openCanvas 1.1. Not very powerful, but usable. Lacks a lot of functionality with its layers (at least it has layers?) and colour editing is pretty much non-existent, but you can network with friends and do a group paint. The program is so old it's easy enough to find a free download. However, SAI is way more powerful and the license fee was so cheap for how often I use the program at home and at school.
I always find Photoshop a good supplement program for the stuff SAI can't do, like move multiple layers at once. Photoshop, being primarily for photo editing, is also way more powerful for post-work than SAI. I do a majority of colour correction in Photoshop, and SAI does not have a text tool, so I also use Photoshop for text. SAI can save and open .psd files, so there is a happy harmony between the two programs, but if you want to use Photoshop... well, it's out there.
I came to this thread seeking some help on my latest work. I need a lot of help as I have come to realize I am not good at painting a realistic face. Oh, I've tried, but they're all just... not... great... It's better than three years ago at least *looks above*
Edit: Updated to the most recent revision.