You can basically make your own "manual fan voltage adjustment device" using a rotary switch and some diodes scavenged from various old electronics. If this thing's wires had not been ripped off in the sea of random parts that is one of my desks and if I had another set of diodes on it, this would be suitable for controlling two fans. It is just a 2 pole 6 position rotary switch with diodes recovered from an old non-functional car tape deck. It originally had wires and connectors recovered from an old failed VCR but I seem to have lost those for the moment.
With the current diode configuration of the pictured switch you have 5 speed settings and an off setting. Very cheap to setup.
I have been meaning to get it a knob and mount it in my case for some years now(that is why it was in the sea of parts and lost it's wires and connectors) but I just never got around to it and my computer's fans are just not loud enough to require it.
For those that must ask adding LEDs to this sort of setup is not hard at all.