The Messenger Library gives developers several value-added features above and beyond the Windows Live Messenger client. One of them is Custom Presence. With custom presence, your site can place its own presence properties on signed-in users. This allows your application to “light up” rich features between multiple users who are signed in to your site.
Here’s how it works: Your site adds its custom presence property to users when they sign into your Messenger Library application. The property can have a name and an associated content object. Then, your app checks the user’s contacts for this custom presence property. If the property is present, the user is signed into your site and can use the special features of the site.
What can you do with custom presence? One possibility is to extend the default status messages (Online, Away, etc.) by adding additional status messages like “Working from home” or “In class”. Or, your users could see status icons with extra “bling” for contacts who are also signed into your site.
To learn more about Custom Presence, check out some of these resources: