Ukulele Pitch Pipes
In the days before computers and digital tuners, pitch pipes were the way to tune up. Ukulele pitch pipes have four holes: each one gives the pitch of one of the strings of a ukulele.
With all the digital tuners around these days, you don't see pitch pipes very often. But they do have their advantages: they're cheap and they never break down.
The disadvantages: you have to use your ears (more difficult if you're somewhere noisy) and they can only be used for one tuning (so make sure you buy the pitch pipe that goes with your preferred tuning).