See this page for a list of companies that provide plug-ins for Premiere Pro, including plug-ins for effects, transitions, importing and exporting specific file formats, audio processing, titles, closed captioning, and workflow tools:
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.
On the hardware compatibility page on the Adobe website, you can find workflow guides for various cameras, as well as documents with details of compatible hardware for GPU acceleration, tape capture, broadcast monitoring, and more.
One of the very big changes in Premiere Pro CS6 is the new Mercury Transmit feature, which makes it much, much easier for third-party hardware developers to write the code necessary to receive video information from Premiere Pro for broadcast monitoring, exporting to tape, et cetera. Check with the vendor for your third-party I/O device to get the up-to-date drivers that make use of Mercury Transmit for Premiere Pro CS6.
If you want to develop plug-ins yourself, see the Premiere Pro Developer Center, where you can download the Premiere Pro SDK and supporting documentation.
Important: There have been few significant changes to the effect plug-in API for Premiere Pro CS6, so you shouldn’t need new versions of most plug-ins for Premiere Pro CS6, if you already have plug-ins that work for Premiere Pro CS5 or CS5.5. There are a few video effects that do require updates; e.g., Boris RED, DVDate, and a few others have needed to make updates to accommodate changes because of the new uninterrupted playback feature. Check the websites of the vendors of your plug-ins to see if you need an update for any reason.
However, beyond changes to the effect plug-ins themselves, 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.