It's called a window regulator guide jaw ($5 @ fastlane). G-benz is right they don't break off completely, but rather split in half. If you look and feel around inside of the the door, you may be able to recover the back 1/2 of the part, the other front 1/2 will still be connected as if nothing was wrong with it.
In my car that part broke and caused my motor to burn out. I ended up spending another $40-60 for the motor.
Change that guide jaw NOW before more damage is done.
jrmd01@yahoo.com