iTunes will do this for you (although another program probably does it in a more straightforward fashion). Just put all your mp3s in one folder, tell iTunes it's the iTunes music folder, then tell iTunes to keep the folder organized (this is all in preferences). Voila! Organized! You can take them out and put them wherever after iTunes has done its magic. If your files don't have artist/album tags, you can use Helium 2 to tag the files properly by taking artist/title info from the file name.
