It is probably easiest to take 8 pin PIC AVR or (insert your favorite MCU with internal oscillator here) nowdays... but:

For fun:
http://www.falstad.com/circuit/e-vco.html

Use suitable RC filter to smooth the input enough... Probably 555 is not that good - bandwidth change is not enough to cover whole RPM range. Still - 99% motherboards do not care about fan speed change range as long there is variable signal present.