David Van Brink (@polyomino) released Omino Python, a free After Effects CS5 plug-in for Mac OS X only (for now) that embeds a Python interpreter:

"This plug-in lets you write a script in Python and then run it on every frame. The script can do drawing, access other layers, and so forth. Because sometimes, you just need to write code to get the job done. Omino Python gives you a straightforward way to provide drawing code directly in your After Effects work flow. This isn’t for everyone; but if you have a little bit of scripting experience, Omino Python should be very easy for you to pick up."

Visit his blog for even more AE filters and resources.

It's not the first time Python scripting has appeared in After Effects -- Adobe once had a Python Plug-in demo in the SDK and Profound Effects had AE plug-in Useful Things that found fans in Trish and Chris Meyer, Jeff Almasol (with Profound Effects, now on the AE team), and others before it was sold to SONY.