Novaskin (http://minecraft.novaskin.me/resourcepacks) is a pretty good browser based one, but it's a little hard to get used to. GIMP and paint.net are also pretty good software based ones, all are free. Doing mine in Novaskin with GIMP as a backup. Novaskin is great because it saves locally until you're ready to zip it, and it actually puts your work on the models they'll end up on, so you're not working from a blank sheet that will get folded into a 3D object later.