According to msn.com here are some thing that you may think are illegal but aren't.

Keep in mind laws vary from state to state and even if the state doesn't have a law making things illegal, there may be some that fall under a local ordinance.

-As of February 2014 it's o.k. to flash your lights to warn other motorists of a speed trap. It was ruled to be a First Amendment right.
-While it's unwise, it's not illegal in any state to drive barefoot.

-Depending on the state, it may be perfectly legal to run a stop sign on private property. In many states, "traffic control devices," such as stop signs, are only enforceable when an automobile is “operated on a street or highway.”

-Using a radar detector is legal in all states except Virginia and the District of Columbia. But don’t try to outsmart the cops with a radar detector if you’re in a commercial vehicle. A federal law prohibits commercial vehicles from using radar detectors.
-Wearing a colander on your head for your driver’s license photo. This one is tricky though. It all depends an the state and their stance on the Church of the Flying Spaghetti Monster and it's members known as Pastafarians.