Here are some sources of more information about pickups

Pickup Manufacturers

B-Band
K&K Sound
Pickup The World
SC Laboratories
Fishman
L.R. Baggs
Sunrise Pickups
Seymour Duncan
Schatten Design
Trance Audio
Highlander Pickups
McIntyre
Taylor Guitars (ES)
Schertler

Other Resources

The Harmony Central Acoustic Pickup Database has lots of user reviews.

This "Acoustic Guitar Resources" web page  has some good links.

Frets.com, run by Frank Ford of Gryphon Stringed Instruments has a lot of great information of interest to guitarists, including some info about installing pickups. Start with A Pickup overview by Mike Gold. good introduction to various types of pickups that are available by someone who's installed a ton of them. And a second article by Mike.

An RMMGA (rec.music.makers.guitar.acoustic usenet group) FAQ on pickups

Santa Cruz Guitar's Pickup FAQ

Shoreline Acoustic Music has some samples of various pickups, recorded by Mike Wong.

Fingerpick Magazine has some really interesting mp3s and video demos of a few  pickups. They're taking a different approach than my comparisons, going to great lengths to get the most out of each pickup, with some significant EQing and effects. It's worth checking out for an idea of what's possible, and Doug Kennedy plays some nice pretty guitar, too.