As Ken Levy mentioned earlier this week, the next beta build of Windows Live Messenger will see yet more extensiblity features being made available to 3rd party developers. Windows Live Messenger Add-Ins can be added to increase Messenger's functionality or to add features. Users will be able to install both official (Microsoft produced) and unofficial (3rd party developers) add-ins, which at a basic level will be able to perform tasks such as changing your display picture and personal message, as well as responding to instant messages and changes in your online status. This is likely to result in add-ins such as automatic translators or those that can chat with your contacts when you're not available, similar to running your own bot in some ways.
To run an add-in in Messenger, you need to have the Microsoft .NET Framework 2.0 installed. Add-Ins can be configured in the Messenger options dialogue, and whilst you can download and install as many add-ins as you like to Messenger, only one can be running at a given time. Also, in order to minimise any possible security risks that could occur whilst running add-ins, Messenger will run them in a restriced environment (a sandbox) to control what they can access on your computer. Having said that, you shouldn't run any Add-Ins you don't trust.
While this isn't something new to those who have used Messenger Plus! or any of the other numerous existing add-ons, this official endorsement from the Messenger team should see a whole new set of developers trying to improve Messenger, which will definitely be good news for users. In order to prove popular though, the limit of 1 add-in running at any time will need to be removed, or at least increased substantially. Information about how to develop an add-in will be available soon via the Windows Live Developer Center.