T-Mobile's really emphasizing the customization with the myTouch 3G, rolling out custom shells, custom gel skins, custom wrist straps, custom pouches, custom holsters and custom wrist straps. But they also have battery extenders (nice!) and docking stations.
The battery extender plugs into the bottom of the phone via miniUSB, and gives you 50% more talk time and supposedly works with "most" HTC phones. The docking station also works the same, but has a built-in mic for speakerphone and a 3.5mm jack to output sound.