The 555 timer is considered one of the most successful chip designs in electronics. It has a simple, well understood, design. The timing on the 555 can be configured to generate a periodic signal using just a few resistors and capacitors. The 555 is often used as a timer in electronic circuits. The Linux scheduler is usually configured to respond to interrupts generated by a cpu. To replace those cpu interrupts by a simple 555 timer is impressive due to the unconventional nature of the setup.
Also, the 555 is an ancient chip, that pretty much appeared as the basic chip (along with it's friend, the 741 general purpose op-amp), being the two of them the protagonists of pretty much 70% of the hobby circuits in the 80s.
