The most popular one is the Nike+ Fuelband of course, because it has the Nike marketing behind it. However upon doing a little research it turns out (not too surprisingly) that it has a cool design, it's actually just about the least functional of several alternatives out there. In other words, pretty, but mostly useless. So pretty much what you'd expect from a major brand like Nike i guess? It certainly doesn't help that it was just recently announced that Nike canceled development of the Android version of the Fuelband app.
I started looking through the list of devices in the "Quantified Self" wikipedia page and basically found that most of them just have an accelerometer. They'll keep track of the number of steps you take per day and how well you sleep at night. Which is great and all, except i've got apps on my phone that can do that for me already.
The one i've found so far that actually does more than that is the Basis device, which i've heard variously described as the "Basis Band" and the "Basis Watch".
Just for reference, here's a roundup i found of a lot of the more popular devices, including the Basis.
The one thing that does worry me is if the Basis Band (or any of the other wrist band devices) will clash with my chain mail bracelets too much. And i mean that in a very literal rather than cosmetic manner :)