"Plug it in and turn your dumb HD tv into a 'Smart'" schenaningans is on the market, if you duck it there are several such USB-sticks, but I dont know how good any of them are. Its more fun making your own I belive, and you get to play with raspberry if you go that route.
Ive tried a Pi model B with XBMC and the output of it is indeed 1080p and looking smooth, it can play an HD file without stuttering/lagging over samba over openvpn. On the openvpn/samba server I have 30mbit up/down and Pi has 54mbit wifi then ADSL with up to 20mbit down.
Hm, if you dont want the Pi route, perhaps getting the cheapest andorid tablet with hdmi out would do well, as you can run a upnp-renderer on it, the tablet should have wifi as well. But then youd still have to run a dlna server on some of your computers, for example on laptop.