Pitchfork

I've just downloaded PitchFork for Mac from

http://www.hitsquad.com/smm/

It's very simple, one small window with two oscillators and a choice of
various waveforms (and sample rates). You input cents, frequencies or
ratios and play either or both of the oscillators. It's certainly
accurate to 1 cent because I tested two sounds a cent apart for beating.
Excellent for tuning up your home made thingies.

Best Wishes.