We showed off Windows Home Server at the Vista After hours sessions last month and there was much interest. What you get out of the box is very exciting, but with any Microsoft platform you come to expect cool things to build on top of it too. This is where Add-ins come in. These are small apps that extend the core functionality and there is a list of the current ones available here.
There's also a competition running at the moment for the best Add-ins built on Windows Home Server RC1 and with a prize fund of over $50,000 it's got to get your interest.