The classic 555 timer integrated circuit, in an 8 pin DIP package. This little chip has plenty of uses. In the MegaSquirt lineup, the older Stim boards used it to generate an RPM signal, and the DIYPNP uses it in its knock circuit. See the MS1/Extra manual for a knock circuit using this timer. Or the Philips Semiconductor 555 application notes for other ideas.
Or use it in any project where you want to show you really could have done that with a 555. It’s the classic electrical engineer’s boast. After all, anyone could have done that with an Arduino if they download enough code libraries.