if you're interested in building your own, there are lots of options you can choose from, but there are basically three different baybus's:
1) you can use a switch to change between preset settings (12v/7v/5v/off)
2) use a rheostat (20-100ohms, generally) to make the fan variable speed. the main drawback is that at the lowest speeds some fans may not start.
3) a variable resistor like #2, but using a voltage regulator you can make sure the fan will always start up. semi-complex to build without a kit, and produces heat.
you can also use a switch with the variable speed baybus's (12v/off/variable)
some links:
bit-tech - several well written articles in "cooling"
hitechmods - well written, easy to follow 12v/off/7v baybus
case etc - their "In-Depth Tutorials"
cliff's fanbus - one of the original resources for fanbus/baybus