Its QUartz Composer stuff, which is indeed much improved in Leopard
Ive found jelly.qtz and putting that in screensaver folder gets it to show up under 'other' but nothing is displayed, probably have to rework where its getting audio from.
Not found out how to turn .qtz into itunes visualizers yet, there was a 3rd party app to do this in Tiger but Im guessing it wont work in Leopard, so I need to go read some official Apple developer docs to see if they support this stuff natively now.
OK the 1 & 2 keys do stuff in Jelly too. And yes, you can write your own visualizers using Quartz Composer, I just tried modifying Jelly (which lives in System/Library/Compositions/ and saved it to /Library/Compositions and it shows up & works in itunes.
So it looks like Screensavers, Visualizers, and probably iChat & photo booth effects are all quartz compositions. But the compositions themselves vary depending on what they are used for, so Jelly would indeed need some work to function as a screensaver. Quartz Composer is fun but gets complex in a hurry so this stuff isnt going to be for everyone, the majority will be relying on others to make quartz compositions and share them. I think the doors are probably open for the R2D2 effect or similar to be put back into Leopard
Alright, so I realize I'm resurrecting an ancient thread here, but I couldn't find any solution for turning Jelly into a screensaver file anywhere on the web. I managed to figure it out though, so I thought I should share the info here just so it's out there.
How to use the Jelly visualizer as a desktop background
Open the Jelly.qtz file in Quartz Composer.
Duplicate and rename the file.
Add a 'Random' patch near the 'Audio Peak (Required)' patch, bottom-left portion of the workspace.
Connect 'Random->Value' to 'Audio Peak->left dot'
Double-click 'Random->Min' dot, enter value of 0.
Double-click 'Random->Max' dot, enter value of 1.
Changes to these values will reflect in the Viewer, so you can preview its behavior.
You can also delete the 'Keyboard' patches and replace them with 'Random' patches, to cycle the presets.
Save, and transfer to Library/Screen Savers folder if necessary.
Activate in System Preferences, then enable your screensaver desktop software or method. I've been using Wallify.