See this page for a list of companies that provide plug-ins for After Effects:
In addition to listing the plug-ins provided by each company, this page provides links to the companies’ websites, so that you can purchase these plug-ins or find how to contact them for technical support or customer service.
Another great place for information about plug-ins for Premiere Pro and After Effects is the Toolfarm website, which provides an online store as well as tutorials, a forum, and other supporting resources for using plug-ins.
The aescripts + aeplugins website also provides many plug-ins (and scripts) for After Effects.
If you want to develop plug-ins yourself, see the After Effects Developer Center, where you can download the After Effects SDK and supporting documentation.
Important: There have been few significant changes to the effect plug-in API for After Effects CS6, so you shouldn’t need new versions of most plug-ins for After Effects CS6, if you already have plug-ins that work for After Effects CS5 or CS5.5.
That said, there are a few changes that have required some vendors to need to update their plug-ins. E.g., RE:Vision Effects has an update to accommodate the new caching system, and Red Giant has updates to fix bugs and such. Check the websites of the vendors of your plug-ins to see if you need an update for any reason.
Separate from the issue of updates to the effect plug-ins themselves, there is the issue of their installers: Some plug-ins come with installers, and (as with CS5.5) these installers may need to be updated to install plug-ins into the correct location.
Toolfarm is maintaining a list of major plug-in vendors and the status of their updates for CS6, and they have some helpful resources for installing plug-ins for which the installers haven’t been updated.