(Announcement by Satisfied Customer, and fellow Arduino user... NOT connected in any way with the source of this £14 (plus p&p) module... I just thought it is a Good Thing you may not know about.)
Need a keyboard as part of something you are building? If you just need to enter digits and a few other things, a 4x4 matrix of pushbuttons is easily done.
But what if you want more... the a-z characters, etc?
Hobbytronics have created a little module. You plug it into a source of 5v. you plug an ordinary USB keyboard (or, if your needs are more modest, a USB numeric keypad) into the USB socket on the module. And whatever keys you press generate serial data which you pass from the module to the Arduino over a single bit. One pin.
Full details at...