Haven't tried it yet, but was thinking a TTL/CMOS clock source could be made using a cheap fLED a transistor, and resistor. Maybe not a permanent fixure in a finished project, but I don't own a signal generator, nor keep a 555 circuit handy (so easy to throw one together as needed). But thought it might be simple and useful, students might find a use as well.
Figured I'd post this first, and see its been done, before spending much time re-inventing. Have a huge list of things to do this weekend (huricane season is starting to get busy...).