Are there discs to help me test, optimize, or show off my audio/video system

AVIA Guide to Home Theater, Ovation Software (extensive video and audio test patterns and setup tutorials)
Video Essentials, Joe Kane Productions (the original system optimization disc, from the master)
Ultimate DVD series, Henninger Interactive (examples of many DVD features, plus test and demo material)
DVD Demystified demo disc (examples of almost every DVD feature, plus demo material)

Here are a few movies that work especially well for demonstrating DVD's video and audio quality:

Dinosaur - Direct-to-DVD digital transfer gives sharp, clear images; good bass on footsteps and fights.
The Eagles: Hell Freezes Over - Outstanding 5.1-channel music (DTS only, Dolby Digital tracks are 2-channel).
The Fifth Element - Excellent video, especially in beginning desert scenes, with stellar audio as well.
Gladiator - Stunning surround audio with brilliantly mixed orchestration.
O Brother, Where Art Thou - Beautiful color and incredible detail (check out facial stubble) with well-rendered shadows.
Terminator 2: Judgment Day (Ultimate Edition) - Great video for shadows and reds; highly dimensional audio.
Toy Story 2 - Perfect all-digital transfer results in sharp, rich images; sound effects are nicely staged.
U-571 - Intense surround effects. Earthshaking bass makes a great subwoofer demo.

Films on Disc has a list of ISF DVD citations -- examples of the best of the craft.