This post will be a bit of a rant because I bought the wrong Western Digital TV streaming media player late last year. Instead of buying the new, latest and greatest, streams Spotify, Netflix and iPlayer (as of today) version, I bought a 3 year old model for the same price. Feel free to laugh in the comments.
This mistake was made all too easily, as they've changed the name from "WDTV HD" to "WDTV Live HD" to "WDTV Live Plus" to "WDTV Live Streaming".
The fact that they're all sold as HD streaming media players means that you have to try to tell the difference between a "WDTV Live HD streaming media player", and a "WDTV Live Streaming HD media player". Not so easy now is it?
They look very similar, apart from the new model having a larger remote (and the extra chipset for streaming Spotify + Netflix, etc).
So you don't make the same mistake I did, here's an official list of models, and here's an unofficial list of model numbers (you might need to register to access the latter).
Luckily I own a Wii which can handle Netflix and BBC iPlayer, but it would've been nice to have everything on the same box, and not feel slightly ripped off.