If a case fan is fitted with a 4-pin Molex male/female pair of connectors, it's an easy and free modification to run it at a lower voltage.
The 4 wires carry +12 volts (yellow) and +5 volts (red) supplies with two 0 volt grounds (black) . The fan leads are normally connected to the yellow and a black wire for the standard 12 volt supply, but if connected to yellow and red the fan sees 12 - 5 = 7 volts.
One way to remake the connection without the need for soldering is
That's it! The connector can still be used to power disk drives or a cd-rom, only the wire colours have changed. Each connector pin 1-4 still carries the correct voltage.
Check the fan starts reliably with the reduced supply, as it could now be very difficult to hear that a fan's not working ;-)
If you want more a bit more control, check out how to switch-operate the two supply voltages in this excellent "how-to" at Bit-Tech.
As a security against the fan not starting at the lower voltage, you can add a capacitor and diode to the 7-volted circuit as described at www.nomad.ee
When the computer is first switched on, the un-charged capacitor is at ground potential. So the full 12v of the supply is initially developed across the fan motor, ensuring it starts.
As the capacitor charges up, the voltage across it rises to about 5.8v (5v plus the 0.8v forward voltage for a 1N4001 diode), when current can flow through the diode to the 5v rail. The fan ends up running at 6.2v. Using a schottky diode (eg 1N5817) with a Vf of around 0.4v, the fan would run at about 6.6v.
The vast majority of fans will start quite reliably at 7v, so this is just a bit of insurance - neat trick, though.