In fact, yes! In general, the Tab Plug-ins—located below your Messenger list (click Add Plug-ins for a list)—are mostly for you, while the Conversation Plug-ins (click the Plug-ins button at the top of a Conversation window for a list) are for doing things together.
For a look at all the possibilities, visit the gallery on the web. Get ready—there’s a lot there! And most of it is designed by non-Yahoo! developers.
Are you the enterprising sort who would like to simply make a plug-in of your own? Great! Here’s a page with information on getting started.