You can find a very nice list of add-ins at the intuitively named whsaddins.com, which carries web-design simplicity to its extreme: a one-page list. Each entry has additional details you can see by clicking the plus sign to expand the listing, and clicking the column headings sorts the list by that column.
According to Philip Churchill, the work was done by Windows Home Server forum user Jacques N. Nice!
Ed Bott (@ wordpress.com) 