There are many options in this category, so I'm sure I will miss a lot.
Your motor controller is a piece of electronic hardware that will takes signals from the computer and convert them to "steps" that drive the motors. Sometimes the Controller and Motor Drivers are all on one board, and sometimes they are broken up into a board that receives the signals (controller) and boards that drive the motors (drivers)
Note - I don't go into Servo controllers on purpose because in my opinion servos are generally out of the price range of most hobbyists
Here's a list of the most popular choices for stepper motor controllers:
Hobby CNC - An all in one board that you assemble, this is a great product if you don't mind soldering and want to save money in this category.
Gecko Drive - A great and very popular choice - the Gecko boards come ready to go out of the box with no soldering or assembly required.
There are a lot of various controllers on EBay and your mileage will vary with them
Making your own - There are many schematics for stepper motor controllers out there - if you are handy at electronics, this may be the way to go