PC Fan Speed Control
To put the comments into an answer:
You can drive most PC fans (which have at least 3 pins) with any PWM source and change their speed through the duty cycle. The Arduino Uno itself has 6 PWM channels. If you want to control more fans, you have to use an extra chip, that provides PWM. You can use the breakout board from Adafruit, but also any other external PWM chip (they are often marked as servo drivers, since servos use PWM signal too).
The speed of most fans can also be changed through providing a different input voltage, but this is not intended for direct variation. Instead this is normally used to set the speed interval, in which the fan should run. For example some PC fans have extra adapters, which lower the supply voltage to let them run more silent. For short term variation you should definitely use PWM. If your fans are too fast at their nominal voltage, you can provide them with a lower voltage permanently.
As dandavis pointed out, the 4th pin is usually connected to a feedback loop. They provide a defined number of pulses per full rotation, so that you can really measure the speed, instead of just estimating. When you don't need this functionality both kinds of fans (3 pin or 4 pin) are good for you.