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If all you're concerned about is over-the-air DTV broadcasts, there are two stand-alone commercial DVRs currently in production in the United States that satisfy:
1) DTVPal DVR
2) TiVo HD DVR

The DTVPal DVR is the economy option at about $299, flat. (That's after a recent price increase; you might find some places still listing the price as $239, but research indicates that none of them have any stock left, they aren't expecting any new inventory and they are not accepting new orders. The device is now a Sears exclusive.)

The DTVPal DVR is a good basic machine. There is a whole thread devoted to it, outlining its capabilities and features:

DTVPal DVR

Note that most of the DTVPal DVR's early failures have been remedied, at this point.

The TiVo HD DVR is the premium device in this space. It should be noted that it is not really intended solely for over-the-air reception. The TiVo HD works with both over-the-air reception and with digital cable, including CableCARD support, so you can even record encrypted cable channels. (The DTVPal DVR only works with over-the-air reception.) So in some ways, if all you ever hope to do with a DVR is record over-the-air channels, the TiVo HD is little like hitting a thumbtack with a hammer. However, if you're ever going to want to record cable television, as well as your current need to record over-the-air channels, the TiVo HD DVR is the only stand-alone choice currently available for sale to the public.

The TiVo HD also offers many extra features unavailable with the DTVPal DVR. For example, with two TiVos, you can move recordings between the two using your LAN. TiVo also offers TiVo Desktop, which you can run on your personal computer, allowing you to pull recordings from your TiVos, and it also contains a server component which allows your TiVo to pull recordings from your personal computer. With extra storage attached to your personal computer, you can see how they could offer you practically unlimited storage of recordings. Along with TiVo Desktop is TiVo-to-Go, which helps you take your programming with you on your mobile devices.

A broadband-connected TiVo also allows you to access Netflix Watch Instantly, Blockbuster Online, Amazon Unbox, and other video on demand sources. (Note that it won't give you access to a cable provider's own on demand options.)

TiVo's UI is considered by many to be the best in the business. It has won not only awards, but also court judgments. (It's so good, others regularly try to steal their ideas.) Their trick-play is fantastic compared to other DVRs, and there are proprietary aspects of the Season Pass and Wishlist features that cannot be beat.

However, there is a big difference in the price between these two boxes. The DTVPal DVR costs $299, flat. The TiVo is sold two ways. The all-in price is about $700. There is also an option to purchase the TiVo (often for about $250) with a service commitment for monthly service (about $13 per month). The TiVo will not work without a valid service contract.