We use Daestra trac plus and it's very good. It sends a position out every few mins or with altitude or heading changes (these parameters are customisable) over Iridium via sms and does not interfere with voice calls. You can login and view all historical tracks too, or view the last 500 on Google Earth, with lat/lon, speed, alt etc.
I find Daestra better than SpiderTrac and Skytrac as, from what I have seen of those two in operation, neither keep the breadcrumbs in google earth - you just get a line. Can't speak for their web interfaces though.