While most Xbox 360 games make using a control pad enjoyable, there are still a lot of people who prefer using a mouse and keyboard, especially on shooters. This peripheral lets you combine the best of both worlds.
The XIM series of controller adapters for the Xbox 360 have been around for a while now, but they've been tricky to use and expensive. The new XIM3, however, seems to have a fix for both those problems.
For one, while previous models were a little...ghetto, the XIM3 is housed in a classy LCD box, which hopefully makes connection and configuration a cinch.
It'll also be cheaper than the last version, which came in at USD$200, because it no longer needs to use specific forms of wired controller to connect to the Xbox 360 (instead just using the standard, official wired controller).
The XIM3 works by acting as a "translator" or "bridge" between your Xbox 360 and PC peripherals. You plug your PC's keyboard and mouse into the XIM3, then plug it into the Xbox 360, and it's good to go.
It of course won't appeal to everybody, especially since so many big shooters get their own PC versions anyway, but if you want to play something like Halo Reach with some more precise controls, this looks like a good way to do it.