Fan controllers

From TheBestCaseScenario

Fan controllers, one very useful accessory, allow you to control multiple fans. They can control up to 6 fans. Each fan controller will have a specific W rating for each line, while decent controllers have a high amount. This allows you to power more than one fan off each line, controlling 2 or more fans together, which is great for groups of fans on watercooling radiators.

Most controllers will only work with 3-pin molex, as some can monitor RPM and either display it on a LCD or through LED brightness/color. Each controller will differ. Some have knobs to change the speed, some have switches (on, medium, high, off), and some have up and down buttons. Some are even self-controlled, or can run off software through a USB connection for maximum control.