Many people already know about this but, There is a way to do a double jump glitch and it's pretty fun just to mess with.

First thing you do is find something to pick up, like a propane tank or a big metal beam you find on top of one of those huge buildings!

Now jump while holding the object and just before you reach full height, throw the object, hit and hold jump again and if you timed it all correctly then you should have successfully done a double jump.

Woot!