In Mac OS X, you can control the position of the dock on the screen (bottom, left, or right), but once you choose a side it always puts the dock in the middle of that side of the screen. But there is a cool trick that allows you to also pin the dock on the selected side so that it's either on the left or right (or top and bottom if you have the dock on the side). Here's how to do it.

1) Open a terminal window.

2) At the prompt, type: defaults write com.apple.Dock pinning <option>, where <option> is replace by one of these three options:

2a) middle -- This is the "usual" behavior with the dock centered on the given side.

2b) start -- If the dock is on the bottom, this moves it to the left side of the screen. If it's on one of the sides, this shifts it up to the top of the screen.

2c) end -- If the dock is on the bottom, this moves it to the right side of the screen. If it's on one of the sides, this shifts it up to the bottom of the screen.

So, an example might look like so: defaults write com.apple.Dock pinning start

3) Once the change is made, you need to restart the dock. You can either do this by restarting your computer, or by opening the Activity Monitor application (should be in Applications > Utilities), finding the Dock application, and forcing it to quit. Once it quits, it will restart automatically.