Use Mp3tag to Organize Your MP3s
Even the most vigilant music maniac can end up with a few dozen MP3s named "Track 1" from "Unknown Album," which is why there are quite a few apps out there that can help you keep your music library tagged, sorted, and properly album-arted. However, we keep on coming back to Mp3tag because it's really good--and it's free (well, donationware).
Just give Mp3tag a directory, and it'll list all your media files for your editing convenience. From here, select a group of files (an album, perhaps) and pick a tag source from the Tag Sources menu--freedb, MusicBrainz, Amazon, or discogs.
You can also pick up album art from Amazon (U.S. or Germany). If the standard ID3 tags aren't enough, you can also edit extended tags (iTunes podcast metadata, for example) in the View menu.
Mp3tag also gets a shout-out for including the oft-ignored Invert Selection option (Control-Shift-A, or choose it under the Edit menu) for those times when it's easier to pick out the three songs you don't want to tag than the 439 that you do.
Bear in mind that Mp3tag is a powerful tool, and you could readily design an organization scheme straight out of High Fidelity. However, if you're syncing your PC's music library to your MP3 player or smartphone, you may find that your reverse-alphabetical-by-drummer's-maiden-name tag system doesn't work too well with a 2.5-inch display. Keep your system simple, and it'll be easier to maintain later.
Have your own automatic media-managing tricks? Share them in the comments!