Many people have the ability to cross their eyes, whether it's by using visual aids or not. It can be a cool party trick if you are around people who cannot, and can be used to generally look mad, cool, weird, etc. There is, however, a way to cross just one eye, which makes for a much more interesting effect. Here are two methods to help you with this:
Find an object in the room at about head height.
Relax, and focus on the object.
Cross both eyes.
Note that you can now see two images of the same object.
This is the tricky part - while keeping your eyes crossed, try and look directly at one of the images. If you can do this correctly, then you will have one eye crossed!
Hold out your finger directly in front of one eye.
Look at your finger with both eyes.
Move your finger in towards your eye, continuing to look at it.
Stop before you poke your eye out!
Now repeat with an imaginary finger...
With practise, you should be able to wobble one eye from side to side while keeping the other stationary.
When choosing an object to focus on, choose one that is about 15 feet away. Also, when you initially cross both eyes, try and cross them as much as you can (but don't go so far that it starts to hurt).