It's worth mentioning that these add-ins won't work if you're running the next-generation of WHS--codenamed Vail. Vail changes the add-in architecture, but you can rest assured that many of the developers who build these add-ins are already hard at work on versions for Vail.
1. Advanced AdminAdvanced Admin gives you access to the key parts of the Windows Home Server control panel from the WHS Console, options that aren't typically available without using Remote Desktop to connect to the PC. Advanced Admin is a must have add-in, especially if you're running your Home Server headless--that is, without a monitor, keyboard, and mouse attached.
2.AutoExit 2009Ever needed to wake a computer from across the continent? With AutoExit and your Windows Home Server's remote login abilities, you can wake and sleep any machine on your home network across the Internet.
3. Lights OutIf you're like me, you've got quite a few PCs on your home network. Lights Out keeps track of the times your PCs are running, so you can monitor their total uptime and power usage. Lights Out isn't free though, it costs $20 for a personal license, although it will run in a reduced functionality mode if you don't want to pony up.
4. TiVo PublisherGot a TiVo? Download TiVo Publisher and access all the media stored on your Home Server from all the TiVos in your house. Photos, music, and video are all available on whatever TiVos you want. And what's it cost? Not a cent, it's 100% free.
5. GridJunctionIf you're serious about uptime on your Windows Home Server box, you'll want to buy a battery backup and install GridJunction on your home server to monitor your system. GridJunction will automatically shut down your server when the power goes out, preventing you from the possibility of data corruption due to unexpected shutdowns or brownouts.
6. uTorrentOccasionally, you've got to download something from Bittorrent. Whether it's the latest WoW patch, a new version of Ubuntu, or "other", the uTorrent add-in for WHS lets you remote into your Home Server and start torrents when you're out and about, so the files will be waiting for you when you make your triumphant return to your house. It's a bit of a tricky install, but you can find a great how-to here.
Do you have a favorite Windows Home Server Add-in? Post it in the comments!