Mechanical Keymodule Producers: Debate and Switch
There are a few types of mechanical switches in existence with the overwhelming majority of designs from ZF Electronics (owners of Cherry Corp.), or Alps Electric. It is actually debatable whether most alternative designs are even mechanical switches rather than specialized versions of other types such as capacitive or membrane. Of course a keyboard such as a buckling-spring Model M might perform as good as or better than a mechanical keyboard, but I would not necessarily call it one.
There are several types of ALPS mechanical switches for keyboards. Unfortunately for anyone attempting to create a guide for them, there is no set specification for their products. ALPS switch designs have changed over time, have been cloned by others, and a categorized by color codes which seem more like a rough set of guidelines contaminated with exceptions.
You could have still used an ALPS switch at some point, however. Guitar Hero controllers have used ALPS-style switches for their strumming mechanism. If you have heard a Guitar Hero controller click at some point, the presence of a mechanical switch should not surprise you. The other popular place to have experienced an ALPS switch is in the old Apple Extended Keyboard before it was cancelled in 1990.
Cherry Corp. Switches
Cherry Corp. has a product line that is very static and very well documented. Their product catalog has nearly complete information about each switch; each switch has a color and style code that is strictly adhered to; and each product has a defined purpose. ZF Electronics even goes to the effort to publish a curve for each switch to state how many centinewtons of resistance you will feel with each fraction of a millimeter the plunger has traveled. It is obvious that Cherry has the advantage in documentation especially when contrasted to Alps.
There are two categories of mechanical switches: the MX full-sized keymodule line, and the ML thin-form keymodule line. The MX line comes in many colors: some are great for certain keyboards, some are designed for space bars; some are common, some are extremely rare and costly. The ML line only has one product, often called the Cherry ML Black keymodule as a result of its black colored key plunger. The ML Black is the switch of choice for the Optimus Maximus keyboard, that keyboard you keep seeing pictures of with OLED screens for every keycap.
Yes I know about the Cherry MY switches…
I consider those membrane switches, not mechanical switches.
The second generation Rosewill RK-9000 offers the choice between the Cherry MX Blue, Cherry MX Brown, Cherry MX Black, and Cherry MX Red.