This page lists some keyboard accessories you can buy that adds extra function keys.
The company fentek-ind.com sells many USB keypads that adds programable keys. One of their most popular is a X-keys strip, that is a row of 16 keys. They also sell other larger keypads that has 24, 35, 58 keys.
This is Excellent! But very expensive. If your keyboard doesn't have multimedia keys, or uses one of those Fn keys that forces you to press 2 keys, you'll want this.
For alternative, i recommend Contour Design's Shuttle Pro 2, see: Why You Need a Jog-Wheel?.
Here's some other options. I think these are actually better, because gaming is a much bigger industry, so the products are cheaper, and you can buy them anywhere.
Though, due to the shape, it is not suitable for normal computer operation. Because, it is designed to be operated when your left hand is resting on it. Unless you are playing games, you can't have your left hand rest on this at all times. To press the key without your hand on it, the device surface is slanted in a way hard to see or use. It's also bulky.
See: Computer Keyboard: Using Numeric Keypad as Function Keys
See: Best Mouse for Programers: 17 Buttons.