There are a number of Content Management Systems that have built-in features like blogging, forums, and limited file uploading.
These built-in features will, of course, share a sign-on.
It is also for these products that you will find "bridge" plugins that let you replace the built-in features with more specialized apps. For instance, Joomla! provides a built-in forum, but there is a plugin that lets you use phpBB as a forum for your Joomla! site with single-sign-on.
I think the way most these bridge plug-ins work is by bridging the user creation process. You then just have to configure the session cookie to be the same for both systems.
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