Video Copilot is now selling a 3D particle plug-in, Element 3D, as well as several 3D model and shader packs to supplement it.
Andrew Kramer has put a lot of work into creating tutorials and documentation for the Element 3D products, and I strongly recommend that you watch and read this material before jumping in:
Lutz Albrecht has a useful post about limitations and technical details of Element 3D, and Rich Young collects resources about Element 3D here.
Note that Element 3D makes use of OpenGL for GPU acceleration, which is also what nearly all of the GPU-accelerated features in After Effects use–with the single exception of the ray-traced 3D renderer, which is accelerated with CUDA. (See this page for details.)
Toolfarm has some good deals on various packages.
Thanks to Andrew and his team for making such a useful set of tools to expand the creative possibilities for all of us who use After Effects.