AppleTV Gen2 or Gen3, if you are heavily invested in Apple's iTunes ecosystem already with your iPhone and iPod. No Amazon on Demand nor Vudu, nor Hulu Plus, nor Pandora, Spotify and other Radio/Music sites, . Very limited selection of channels and apps because they drive you to iTunes, although it can be hacked, and Plex or XBMC loaded. Airplay is a nice feature which integrates your Apple devices.
Roku Box, if you want to maximize content availability. This one plays most all except Vudu.
Boxee Live TV, if you are using OTA as it integrates OTA channels into it's On Screen Guide. No Amazon on Demand nor Hulu Plus. Boxee;s latest streaming box, looks very promising and is expanding content.
Netgear Neo TV is worth a look too. It's most like Roku, but doesnt have the amount of content to match it yet. Roku is better all the way around still though.
Bluray Players with Streaming Ethernet/WiFi are another way to go, but are generally more limited than Roku, but have improved much in available Apps/Channels.
Other than that there are laptops and dedicated HTPCs which allow maximum versatility and choice, but require more hands on work and knowledge. Or you can get Media Streamer boxes which also require more knowledge of complex computer workings for DNLA use but also add Streaming Channel simplicity on top.....like the WD Live series and many others. Google TV which is on the Logitech Revue is likewise very buggy and complicated.
Bottom line if you are looking for ease of use and streaming content availability then
AppleTV if you are heavily invested in the Apple ecosystem already.
BluRay Players with Internet Streaming Ethrernet/WiFi
are your best bets.
Boxee Live TV isnt quite there yet, but I predict is will be a real competitor to AppleTV and Roku within a year.....especially for OTA Broadcast cord cutters, but it also integrates Cable into it's guide as well. Keep your eye on this one.
Boxee Box was too complicated and tried to do too much like Google TV and WD Live, IMO. So it was always a niche product for nerds and enthusiasts.
Panasonic AX-200u / Optoma Graywolf 92" / Draper Luma 92"
Samsung 51F8500 Plasma / Panasonic P50ST60 Plasma / Sony 50W700B LCD / Vizio VP322 Plasma
Sony S790 BD / Panasonic BDT-220 / Roku 2 XS & Roku 2 (Ver. 3)
Panasonic S97 / Panasonic RP-91
Aiwa MX100 Multi-region VCR / JVC S7600 S-VHS