Just right after Tiffen and Digital Film Tools announced their new image editing plugins for Aperture 2.1 Apple has released the Software Development Kit (SDK) for coding Aperture 2.1 plugins. It’s available from Apple’s Developer Connection for registered members (registering is free). You can grab the Aperture 2.1 plugin-SDK (3D9) as a 595KB download from there and start coding.
If you want to know what exactly you can do with it as a programmer you should read those lines from Apple.
So it should be just a matter of time before we see another bunch of plugins popping out.