Resources

Normally your media player will supply the relevant information.
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.

A Google image search often retrieves missing cover art.

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

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 - Petrucci Music Library

Lots of information about composers and their works.

Over a 100.00 scores and even recording.

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