A while back I found the need to have HTML checkboxes behave like radio buttons. As you probably know, the main difference between the two is that multiple checkboxes can be selected at the same time whereas only one radio button can be selected at any given time (of course this changes depending on the context).

So how do you make checkboxes behave like radio buttons? With a bit of Javascript, it's pretty easy (click "Result" to see it in action):

As you can see, only one checkbox can be selected at a time; the checkboxes are behaving like radio buttons! This solution allows you to leave both boxes unchecked, or check only one.