ExtUSB is an HTC proprietary connector that features a USB (with power) and Headset connections in a single combined connector. It is backwards-compatible with USB, but has extra 6 pins to support audio. The pinout is located here:
http://forum.xda-developers.com/showpost.php?p=1063267&postcount=1
An updated version including support of video output on the Raphael can be found here:
http://forum.xda-developers.com/showpost.php?p=2610305&postcount=19
Currently, the following HTC devices have this connector:
(* The Blackstone/Touch HD has no audio/headset on the ExtUSB, the sense pin seems connected, but triggers a 'not supported' message)
http://forum.xda-developers.com/showpost.php?p=1063267&postcount=1