Depending on what you want your keyboard for, there are a variety of different switches that you can choose from to fit your needs.
Right now, almost all mechanical keyboards use Cherry MX switches. There are many different Cherry MX switches. The four main ones are Black, Blue Brown, and Red. Each one of the switches has its own feel.
Cherry MX Blacks are switches designed for gaming. Requiring an average of 60 centinewtons(60cN) of force to activate, they are slightly stiffer than most keys. They do not register a button press until the key "bottoms out", or hits the bottom of its possible degree of movement. These switches have no tactile "bump" or audible click to determine a key press. These are called "Linear" switches.
Cherry MX Reds are a less stiff variant of the MX Blacks (45cN), and still have the same linear feel. They also require you to bottom out the key for it to register the keypress. This helps prevent unwanted frustration of nudging a key in skyrim and dropping your best weapon.
Cherry MX Blue switches are a "clicky" form of switch. They do not require you to bottom out the key to register a keypress and instead create a tactile "bump" and an audible click to indicate that a key being registered. Requiring 50cN of force to activate, they fit right in the middle for key stiffness. This type of switch is recommended for people who use their computers for typing as it is easier to tell when a key registers than the blacks and reds and it can help speed up typing because you learn to hear and feel the "click" instead of bottoming out your keys.
Cherry MX Brown switches are a slightly less stiff (45cN) tactile-response switch. It acts like a Cherry MX Blue in the concept that it does not require bottoming out, but instead only provides a tactile "bump" instead of a click. This is a great keyboard if you like the tactile response of the Blues, but you don't want to wake everyone up because you are writing an essay at 3:00 in the morning.
See http://deskthority.net/wiki/Cherry_MX for more information on the other types of Cherry MX switches.
Those are the main types of Cherry MX switches. Higher end mechanical keyboard brands like Filco and Ducky all use Cherry MX switches.
The one other major type of switch you will hear about besides the Cherry MX switches are the fabled "Buckling Spring" keyboards. These guys are famous. Being first included on the famous IBM Model M keyboard, it has remained a popular switch for many years. People still are using model M keyboards to this day as a matter of fact. Buckling spring keyboards are used because of their reliability, durability, sound and feel. When getting a Buckling Spring heyboard, you are not just buying the switch, but the history that comes with it. The stigma of owning one, its reliability, and its rarity are all contributing factors to its place of being declared "the king of switches".
Most of it is hype though f you ask me. I have never used one myself, so i can't voice the opinion on that one too much. Most manufacturers do not use buckling springs any more, Unicomp being one of the few remaining ones.
NOW FOR MANUFACTURERS: Many manufacturers now make mechanical keyboards, but they are of varying quality and design.
The very baseline mechanical keyboard manufacturers (ie. Razer, Corsair, Logitech, etc.) usually do not make the keyboard fully mechanical. they may make only the letter keys and a few others mechanical and leave the rest membrane switches. The keyboards also tend to be quite flimsy and use gimmicks and marketing tactics to sell their keyboards. These can cost anywhere from anout 50-80 USD.
The mid-range keyboards (Filco, Ducky, Unicomp) use all mechanical switches in their keyboards and tend to be a bit sturdier built. These keyboards can go fro about 100-200 USD.
The top-end keyboard manufacturers use special "Topre" (pronounced Toe-Pray) switches that are designed to prevent "pvssyring", the registering of one keypress as multiple, and produce a very unique sound and feel. They are all produced by Topre corporation. They have a few different models of keyboards that can run up to 400 USD for one. Very expensive, very well made, highly collectable, and most likely not worth the price tag. (They are essentially a refined membrane switch.)
For this purpose, I will assume that you are looking at a mid-range board. Depending on what you want, I would recommend these 3 brands.
Filco's Majestouch keyboards are very popular, very reliable, and possibly the heaviest and sturdiest boards in the market. They are so strong that people joke that they could use it for home defense. They also are right about the perfect price for a ncie board, usually going in the 110-180 USD range. Their one downfall is none of their keyboards have built-in media keys or backlights.
If you want backlights, media keys, and an absurd number of other features on a keyboard, look no further than the Ducky Shine series. The Ducky Shine 1, 2, 3(soon to be released) models are jam-packed with features, gadgets, and doodads. The downside is they are not well built when compared to others at the price point and they are also on the more expensive end, clocking in at around 150-220 USD per board,
Unicomp is the rebel of this group by going at an absurdly low 80-12 USD for one of their buckling spring boards. They are pretty much IBM Model M boards with Windows keys.
Well, that is just about all i have to say on that end. This turned out a lot longer than I expected too. I should probably go to bed now.