I also think that Hank Levin's Inner-Trac should be listed here.
While he is marketing it to EFT practitioners, I have used it in a variety of Auditing procedures ranging from Book One intro sessions to Dianetics R3X sessions. Others I have been in Comm with have used in a solo capacity as well.
The device itself is very much like an E-meter just without a voltometer needle or dial. It certainly is a GSR biofeedback device like the e-meter is as well.