Thanks dc42, I know of those pwm-capable fans, but I'm prototyping and I already have a bunch of fans laying around from a previous project and I'm trying to not blow my budget with buying things I already have. Any thoughts on how I can just switch the voltage going to them? I'm using a PC power supply, so I'll have 5V and 12V readily available and it would be great if I could switch between 5v and 12v rather than doing a variable control.
** I thought brushed motors were more difficult to control through PWM than brushless... Maybe they are brushless then, I'm not sure. I just remember that I tried it before and they didn't slow down or respond to the PWM control through the motor controller at all.