I semi have one. Its mainly just used for network storage and a few other services but I recently installed boxee on it.
As far as programs that will let you make great use of your htpc on you tv boxee is a great choice imo. It searches your network and many different places on the web for content such as youtube, hulu, and netflix. Althought for netflix you need to have it running on either OS X or Windows, as microsoft does not support silverlight on linux.
I have boxee running on Ubuntu with a old amd chip, 1g of ram, and a $30 vid card, and its more than enought. I don't know how well those specs would hold up if I had to run win7 or something on it though.