Resources

Normally your media player will supply the relevant information for the tags using an internet database.
But sometimes it fails you. There are a couple of alternatives.

Tagging

MP3Tag is a powerful piece of freeware.
Beside FreeDB it can use amazon.com, MusicBrainz and Discogs for tagging.

Cover art

A Google image search often retrieves missing cover art.

Information

Wikipedia

A lot of information about composers and performers often including a discography can be found here.

All Music Guide

A well-structured source of information.
Good source for classical music.

If you look up Beethoven you get over 700 entries including all WoO (Werke ohne Opuszahl).

 

IMSLP (International Music Score Library Project)

Lots of information about composers (7000) and their works (55.000) in the Petrucci Music Library.

You can even search by melody!

Now if I would have know how to play the keyboard....

Hits

Longbored Surfer took the trouble to publish all Billboard top 100 charts from 1946 on.