I use photoshop, because I was able to get it through work.
For some things, however, notably adjusting contrast (outdoor photos when the subject's face is in the shade) Picasa, available free from Google, works so well, it's almost spooky.
One big advantage of Photoshop, besides the fact that it will do, well, everything, is that there's a monthly column in Popular Photography that explains all kinds of nifty techniques.