There are many, however most modern one have an issue or 3.. First you are sitting in front of one just now.. You get the WIN-TV adapter (Replay edition is best) for your computer and it becomes a DVR.. one of the best in fact. assuming it has the oomph for the job and is a compatible Operating System.
But failing that.. There are many I just did a quick Yahoo search and found a whole bunch including TiVo and many other makes and models.. I'd check with Best Buy first, or buy on-ine.
NOTE: What I'm not sure of with any product, is the ability to record HD in HD... My DVR's are older (2002) and do not do HD, they do, however, record OTA Digital in SD via an adapter box. and cable as well. They also automatically select the source (Cable or OTA) and control the adapter boxes, and "Chat" so I can watch any recorded show on either DVR, or download and take with me on a portable device.
Just like the "Only our Expensive over priced, ripp off Subscription service lets you do all this" adds.. I do it, 25 bucks a year (For the Electronic program directory so I don't even have to program in all the shows.)
But, alas, they don't make 'em any more. TiVo is the closest you can get.