One of my favourite things about virtual worlds is the ability to integrate other applications and communicate with the external web.
One of the nicest applications that I have used so far is sloodle also known as the simulation linked object oriented dynamic learning environment. This is an open source system that allows you to communicate with an external moodle application, a course management system also open source.
The sloodle team have been working in the SL grid for a long time but in the last month they've been porting it to work with opensim. They have been busy with the port and packaging but I cannot wait to test it!
They also have other nice projects as the sloodle browser.
If you need an intro to sloodle, there is an introductory class every tuesday at 1.30pm U.S. Pacific time in the sloodle clasroom.