This Monday night we released the beta for Yahoo! Messenger 8.0. This release has the usual set of UI improvements and feature changes here and there, but the big news is that Messenger is now opened up for developers. We’ve added support for two types of plug-ins: Tab plug-ins load in the lower part of the main application window while Conversation plug-ins load on the side of a 2-way IM conversation. The Yahoo Developer Network has all the details including the Messenger SDK. We’re also part of the beta Gallery site which will host plug-ins written by Yahoo and 3rd-party…