Oddly  yes enough we have ( amongst others) the Spalsat 'Tracker' system
http://www.electrifyingtouch.co.uk/information/car_alarm/tracker.htm where
you have Lojack ( I understand a UK company is developing it for use over
here) . Our alarm systems which our insurers require are Thatchem approved
Cat 1 alarm immobiliser and Cat 2  stand alone immobiliser( both have to
fitted by a certified installer) I would expect US insurers would require a
homegrown approval. If you dont need an approved device Ebay abounds with
feature packed japanese alarms. I fitted  a pretty good one with dual
circuit immobiliser voltage drop detection ,shock, ultrasonic perimiter
sensor and internal pressure sensor tho' the perimiter unit was not very
reliable ( Cat + German Shepherd+ Spaniel  issues )
I would be looking for
1) self arming - with disable (valet ) otherwise maintainence is a problem
2)battery backup-in case the thief attempts to disconnect
3)2 circuit immobiliser usually starter and fuel circuits
4) internal sensor - volumetric or ultrasonic
5) 2 stage alarm so an accidental nudge does not trigger a full alarm event
6) avoid voice warning systems unless they can be muted my neighbours were
not impressed by the phrase "system disarmed" when I was on early duty.

Derek