It's a level-triggered retriggerable monostable multivibrator with inverted output.
(Old: Ouch 90mA in! Only a very brief spike though. Slow it down for a closer look, and ... 250mA for about 20us! That's with 500nF, 1kR, 500R.) Much better with much smaller cap and larger resistors: 25mA for 20us. If I remember right, that's ok for 74HC (but check the datasheet. :)
