Toolfarm mentioned a new Mac-only product, Effect Builder AE. Pixlock explains:
"Effect Builder AE is our development kit for building Adobe After Effects™ plug-ins from Quartz Compositions. With Effect Builder AE and Quartz Composer you can quickly create your own effects like generators, filters, and transitions without programming knowledge. With Core Image enabled graphics hardware, effects are rendered hardware accelerated."
Now someone should be doing something similar with Adobe's Hydra/AIF, which could possibly cover AE, Premiere, Photoshop, and Flash.
Update: QC Integration FX offers something similar -- an ability to integrate any Quartz Composer composition as a plugin directly into Final Cut Pro and Motion. QC Integration already includes 52 Quartz Composer compositions, all fully usable as high quality visual effects plugins for Final Cut Pro and Motion.