Brake piston tool?

[rhock99 at epix.net (Ryan H)]

I'm trying to back my brake piston into the rear caliper for new pads, and I've got a "non-factory" style brake piston, where instead of a hex hole in it, it has a wide (1/2"), straight slot across the entire piston.  Even my widest screwdriver doesn't do much of anything.  What kind of tools do people use for these?