I'm a big fan of the phrase "Just Kidding". It's a way to say those thing that you really mean without offending. A sarcastic truism that has no sting because you've rendered it void with two simple words.
And it's not just for the sarcasm you mean. It can be for the sarcasm you don't mean, or it can represent the opposite of what you are thinking. In fact, there is no humor-filled instance when the phrase "Just Kidding" cannot be used.
However, just because you can say it, doesn't mean you should. There are several instances when saying "Just Kidding" is wrong, and potentially illegal.
For example, shouting "I've got a bomb" in a busy airport followed by "Just Kidding" probably won't keep you out of trouble. The "Just Kidding" defense isn't admissible in court. Also, telling your serious girlfriend/boyfriend "I love you...just kidding," could quickly end a relationship. And tagging a "Just Kidding" onto the tail end of a funny joke actually makes the joke less funny.
So let's all give laughing round of cheers to "Just Kidding". After all, he who laughs last didn't get it.