I've gone through and tested all of them to find those that would work, and removed a few that just wouldn't work right or were just plain boring. The list of 24 I ended up with add three panels of visual effects to both iChat and Photo Booth. The list is as follows:
ASCII Art, Blue Print, Blur, City Lights, Color Controls, Color Invert, Compound Eye, Concert, Copy Machine, Crystallize, Dot Screen, Exposure Adjust, Film Stock, Gamma Adjust, Kaleidescope, Line Overlay, Line Screen, Monochrome, Neon, Pixellate, Pointillize, Posterize, Sharpen, Swing, Tracer, Zoom Blur
To make these files work, you first need to drag them from /System/Library/Compositions to /Library/Compositions. If the Compositions folder doesn't exist in /Library, simply create it. After moving the files (or those you'd like to use), drag the files from the new location into your favorite binary-property-list-aware text editor, such as BBEdit, TextWrangler, PlistEdit Pro, or Property List Editor (part of Apple's Xcode developer tools package on the OS X disc.).
Locate the following text within each file and delete it:
<key>excludedHosts</key>
<array>
<string>com.apple.PhotoBooth</string>
<string>com.apple.iChat</string>
</array>
Save the files. When complete, you will see three additional groups of effects available in both programs (if you edited all 24 effects). Some are quite unique and others a kinda cool. I've been doing this since Mac OS X 10.4 (Tiger), and it still works in 10.6.x (Snow Leopard).
[robg adds: In Snow Leopard, you can see a preview of each effect by using Quick Look, the Preview column, or CoverFlow mode. If you're using Property List Editor or PlistEdit Pro to edit, delete the excludedHosts key. I tested this one with the Tracer effect, and it worked fine. ]

