A good keyboard is worth as much as a good monitor: because you should be spending the most on the part of the computer you actually touch. Computers have now been flooded with lackluster rubber domes, but gaming enthusiasts have revived the concept of the the premium Mechanical Keyboard.
Look for a good ol' mechanical keyboard, that clicks with magical tactile feedback like the typewriters of old. Very important if you type a lot or game a lot.
- Rubber Dome - The cheap, muted keys that have dominated the landscape, and are standard on every laptop. You can probably get the worst ones for a buck or two. The best one of the bunch is the Thinkpad keyboard, a serious business typing machine that tries to bring back the classic feel of the IBM Model M, and even has a pointing stick built in. This keyboard comes with a Thinkpad laptop, and a standalone USB version for desktops can be procured for $30.
- Buckling Spring - The IBM/Lexmark Model M and Unicomp keyboards. These keyboards have a spring that snaps against a conductive wall, giving it unparalleled durability, a typewriter-style feel, and the classic loud clacking sound of a true keyboard. It's great for typists, programmers, anyone who spends a lot of time on the keyboard (it might be too clicky for gaming, though). They just don't make them like they used to.
- Mechanical switch - Almost every modern mechanical keyboard uses Cherry MX or Alps switches. Although they're not the same gang for gaming, last longer, and are a cut above the average.
- IBM Buckling Spring Keyboard
- IBM Wheelwriter - The IBM Wheelwriter surprisingly is equipped with a buckling spring keyboard, the same switches as the Model M. In fact, it can be set up to work on a modern computer with a keyboard controller.
- Apple Extended Keyboard II - The best Apple keyboard ever made, with Alps mechanical switches (similar, but not the same as modern Cherry).
- Compaq Pocket PC Keyboard - An interesting butterfly keyboard that fits in a pocket.
If you're making or repurposing an old keyboard, you will need a keyboard controller to convert those electrical signals to USB input. Also, if you need a keycode converter (PS/2 to USB or AT to USB), a converter is what you need.
The Teensy microcontrollers with Atmel chips work great for this purpose. Also Arduino Micro for small keyboards/converters on the cheap.