I recently saw this thing: https://www.simple.tv/
In general, the sticking point with most OTA DVR solutions is that, in order to be "free", they have to get their program guide (EPG) info from OTA signals. That typically means you get 2-3 days of info, tops. That's not very effective in terms of being able to set a show to record easily - especially if you are, for example, going on a few days' vacation. Some of the various devices out there offer free EPG if you are okay with three days, but offer a pay-per-month deal (typically cheap, maybe $5) to get full two weeks EPG data from them via phone or internet connection.
Personally, I have SageTV, which, from all I have read in the last 4 years, was the best HTPC software in terms of reliability, expandability, and the ability to watch anything and everything. However, you notice I said "was" - because SageTV was bought by Google last summer and is now (at least for the time being) dead. It still works for those of us who have it, but you can't buy it anymore if you don't.
However, there is a little hope that Google will do something with SageTV. See a post from the owner/CEO of SageTV, who is now a Google employee: http://forums.sagetv.com/forums/show...2&postcount=20