I would love to see someone create a Folding or SETI add-in. Of course I run bittorrent all the time so my bandwidth and cpu cycles are pretty much in constant use...but I think a lot of other people would enjoy running these distributed computing clients to make use of their hardware.
Oh duh. I feel silly now. That would work for folding too. I haven't actually installed either on WHS though so I can't really make any suggestions as to whether or not an add-in could improve the experience. Maybe it could be configured to show the graphical display of the active computing job?