FreeRide sometimes has ridable used bikes in their "library", which you can borrow if you put down a deposit. As far as I know, the system of charging $20 for the deposit for any bike is still in use. I would ask someone there to look over any bike before you ride it - to make sure everything is tightened that should be, brakes work ok, etc.
You get your deposit back when you return the bike, which doesn't have to be same day. Just ask about getting a library bike.
They're open saturday* from 1-5, inside construction junction, http://freeridepgh.org/
*sorry. edited. originally said "tomorrow". For some reason, it's friday in my head.