Quartz Composer is a lot of fun. Tonight I did a little ST-Demo look alike with it. On my G5 it runs with 13 fps in full resolution, so it might be a bit overpowering for most machines. So download Mulle Zoomer give it a try and throw it away. It's nice, but it's not that special...
Next up, that objc_msgSend thing revealed, as much as possible under NDA.
Check out Pol-Online.net and have a look at Pixelshox Studio. This is a great application to create interactive visuals, which has been discontinued... Thanks to Stephane Sudre for this tip.