I. Introduction
II. Terminology
III. Switches
IV. Key Types
V. Printing Types
VI. Keyboards By Type
VII. Sources
Introduction
Mechanical keyboards have been used since the beginning of computers, and predate membrane keyboards. Reliable and solid, Mechanical keyboards tend to last a great deal of time for the average user, more so than membrane keyboards. This, however, comes at a trade off. Mechanical keyboards are an expensive and thought intensive purchase, requiring proper research and knowledge to make an informed purchase and to be satisfied with your product. Mechanical keyboards can be purchased for a variety of reasons. Reliability, features, and feel are all important factors that determine the success of mechanical keyboards.
Terminology
Switches The vast majority of switches you'll come in contact with are developed my Cherry, a German company. They come in a variety of "colors" which define the attributes of the switch. More on this later.
NKRO/#KRO NKRO or KRO stands for Key Roll Over. Basically, it defines the amount of key presses the keyboard can take. Most Mechanical Keyboards take advantage of a high KRO for typing and gaming, so you have a flawless gaming experience. Most keyboards that have a high KRO will interface through the computer with the PS/2 port, as it takes complete advantage of unlimited KRO. Most common USB keyboards have 6 key rollover.
Tenkeyless A tenkeyless keyboard is a keyboard without the number pad for a more compact keyboard.
Tactile A click feedback can be felt in the key from actuation.
Clicky A click can be heard from the actuation. (And not from the key bottoming out.)
Switches
Cherry MX Blue
A common typing switch, the cherry MX blue employes a tactile and clicky non linear switch. It's also found in Razer's "Blackwidow" series of keyboards. (Note: The Non-Nina/Stealth/Silent version.) These are loud switches, and not recommended for a public workplace. Additionally, if you're running audio sensitive setups such as streams or ventrilo, there's a good chance these can be heard over your microphone.
Cherry MX Black
A common gaming switch, the cherry MX black employes a linear, non tactile, non clicky switch. They have a significantly higher actuation force, and are great keys for games that require multiple, quick key strikes such as RTS and MMOs. (Key spamming.) They are not recommended for typing due to their linear nature.
Cherry MX Brown
Another common typist switch, the cherry MX brown keyswitches employ a non-linear, non-clicky, tactile switch. They are used commonly in keyboards where the tactile feel of MX Blue's would be desired, but the sound would not. (Many "silent" Mechanical keyboards use Cherry MX Browns.)
Cherry MX Red
Another gaming switch, the cherry MX Red keyswitches are linear, non-clicky, non-tactile switches. They're incredibly sensitive, and not recommended for typing on. Cherry MX Red's are an acquired taste, due to their incredibly low actuation force. (Many "First Person Shooter" keyboards, such as the Corsair K60, use MX Red switches.)
THERE ARE OTHERS
Buckling Spring, Cherry MX Clear, etc. All of these exist and can still be found, but are very unpopular for various reasons. I strongly recommend staying away from any mechanical switches that are not listed above.
Key Types
There's a few things that determine the "type" of keys.
Flat Common on laptops and chiclet style keyboards, you'll be hard pressed to find a mechanical keyboard with flat style key caps.
Spherical Common on older terminal style keyboards, these have lowered middles and raised corners, to fit the tip of the finger. These are more comfortable when struck directly from the top.
Cylindrical The most popular key type by far, these used raised left and right sides with a slightly lower middle to better fit the fingers. There's a good chance the keyboard you're typing on right now has cylindrical key types.
Printing Types
There's also various types of labels for keys. Some can be successful, and some can detract from the keyboard.
Pad These key printings can be felt on the keyboard, and are similar to tiny labels or stickers on the keys. These are cheap, and can scratch and wear off over time.
Laser Etching Laser etched keys are commonly found on white or light colored keys. This is because when the plastic burns from the laser, it turns black. These are common on older keyboards. When done on black keyboards, sometimes a dye will be used to fill in the cracks of the laser etching to make it visible. (Other times, people just desire them for the feel.) These can be felt on the keys.
Dye Dye [Sublimation] uses a dye which seeps into the keys and dyes the plastic. This is an expensive process, but will not wear away, and cannot be felt on the keys. It cannot be used on dark keys.
Injection Molding A small colored key is placed in a larger normal sized key. The small key is raised with the insignia or key design desired, and comes up through the bottom, and levels with the top. This cannot be felt, is the most durable*, and most expensive.
*However, as the key ages, the two plastics can separate and the edge can sometimes be felt.
Keyboards by Type
Cherry MX Black
Steelseries 6G
Steelseries 7G
Mionix Zibal 60
ThermalTake Esports Meka G1
ThermalTake Esports Meka
Filco Ninja Majestouch-2
Rosewill Rk-9000BL
Cherry MX Red
Qpad MK-50*
Qpad MK-85*
Steelseries 6G Red Edition
Corsair Vengeance K60
Corsair Vengeance K90
Rosewill RK-9000RE
Cherry MX Brown
Zowie Celeritas Pro
Zowie Celeritas
Razer BlackWidow Stealth Ultimate
Razer BlackWidow Stealth
Filco Majestouch-2
Filco Ninja Majestouch-2
Das Keyboard Model S Professional Silent
Das Keyboard Model S Ultimate Silent
Rosewill RK-9000BR
Cherry MX Blue
Razer BlackWidow
Razer BlackWidow Ultimate
Qpad MK-80*
Das Keyboard Model S Ultimate
Das Keyboard Model S Professional
Rosewill RK-9000
*All of the Qpad Keyboards are currently available with all switches from their Online store, you're free to customize pick whatever you prefer! - enzity
Sources
Cherry Corp MX Key Switches
Overclock.Net Mechanical Keyboard Guide