Shooting Holes in Wounding Theories: The Mechanics of Terminal Ballistics - A thorough and in-depth site that explains a lot of what happens with various sizes of ammunition. Very well written.

American Handgunner - Article: Penetration Testing Big Bore Handguns - "A handgun isn't all that far behind an Elephant rifle"

Bullet Penetration Testing - Compares newspaper, water, and actual results from a deer. With all of the images, you can see the different rounds before and after they were fired into various mediums.

Walls resistant to bullets - Thorough testing of wall materials and how they resist various caliber bullets.

rec.guns FAQ - It's always a good idea to know where one should go for answers.

Simplified 10% Ordnance Gelatin Preparation and Test Procedure - Very organized instructions with pictures to help out with the steps.

Ballistic Gelatin Recipe - From the rec.guns FAQ.

Bullet tests report - PDF formatted report with images and tables for penetration tests.

FBI bullet penetration tests, in PHP, sortable - It's pretty nice to be able to look at the information in this manner.

Numbers about bullet lethalness, formulas, etc - A well-written article that shows how proper ballistic gel should be prepared and tested.

FBI Penetration Testing - Explains how gelatin blocks are used for the FBI's ballistic testing.