Movies

The Internet Movie Database - Thorough online database of almost any movie you can think of, no matter how bad
The All Movie Guide - From the same people who do the All Music database
Angry Alien Productions - See your favorite films reenacted, in 30 seconds, by bunnies...
Netflix - Watch movies online
Metacritic - Keeping score of entertainment
Rotten Tomatoes - More ratings and reviews

TV

The Simpsons Archive
epguides.com - Episode guides to a huge number of past and present TV shows

Theatre

The Internet Theatre Database - Limited so far, but attempting to duplicate the Movie Database's efforts for Broadway
The Internet Broadway Database - Another one, a little more complete
Broadway.com - Official Broadway supported site

Music

General

The All Music Guide - Comprehensive online rating and biographical guide
Wolfgang's Vault - From the archives of the great Bill Graham
Pandora - Free internet radio

Genesis

The Genesis Discography - HUGE document, contains almost any info you could want
Genesis-Midi.com - Source of MIDI files for Genesis songs