If you want something simple with a built in hard drive I would recommend looking into the WDTV live hub. The interface is reasonably good, it plays everything I tried and also plays online stuff as well. Also if you leave it running when not in use it has a media server built in, so you can play the media on the hard drive with other devices that support it (PS3, Xbox, other players).
I've heard the patriot box office and asus oplay mentioned, the patriot played everything I tried and you can buy the drive separate to get the space you need, but the interface wasn't that great and I don't remember what it had for online content. Never tried the ASUS.
Otherwise you can plug an external drive into most of the common streaming players. They are a bit cheaper, can do local network playback and may have a few more online services if you plan to use any of that.
If you want to make sure it can do everything, I've put together Fusion based HTPC's running Linux for about $200 and running windows for around $300, although I didn't bother with an optical drive for either.