They were routinely sold under the name "MacCracker" - and had a flat disk with a ridge. Once the screws were out, the ridge would be worked as a lever to separate the edges.
I made my own, I got a torx bit set and an extender. The extender would not fit all the way, so I spent an afternoon filing the extender down until it was narrow enough.
http://www.cultofmac.com/122408/signed-by-steve-jobs-co-sign...