Homemade keyboard with microcontroller
It really depends on your experience with microcontrollers and also budget you have.
If cost is no issue, any USB capable MCU will do.
If you want to do it as cheap as possible, you can use some AVR MCUs even without HW USB. You can use this library VUSB, which creates SW USB on your AVR device.
You can make your keybord electronics below 5$ with such setup.

Here are some articles about using this library:TutorialHackADay

I'm sure there are similar libraries for other MCU's from other manufacturers.