IMO, from reading about the capabilities of thee boxes, seeing interface screenshots, the best media extender product at this moment is the Western Digital TV Live. Full HD decode and display to 1080p. I can't think of any product that comes even close to it frankly, including any of the ones mentioned in this thread already.

It's wired, but it supports connecting a USB WiFi adapter.

It will play back more formats that most other extenders and has one of the better interfaces. Most products on the market, including the Popcorn Hour are extremely poor in this last regard. The PS3 doesn't support many video formats, so it's generally a non-starter. The XBOX needs something like Boxee or XBMC installed or it's useless. Both it and the PS3 are also loud and failure prone. These dedicated extenders are nearly bulletproof, silent, cool, low-powered and tiny. Very easy to integrate with the rest of your kit.

The WD box is also one of the cheaper alternatives. I'm connecting up one of my friends with it in the next month or two.