Car Games!

Follow the leader:
........With Friends:
This is simple, the person in front goes one way, signals one way and everyone tries their best to mimic it as exactly as possible down the chain. This can be kind of fun, like an all school hug meets telephone meets automobiles.
........With The Unsuspecting:
Kinda creepy...sense this is basically stalking someone till they get off of a road.


Leap Frog:
Requires: A two lane road, or dashed yellow line on the proper side.
........With Friends:
The rules are all of you get in one lane, and drive the speed limit amongst each other, then the person in the back must attempt to pass you all. Take turns until this becomes too reckless. Also, the people forming the chain, must be close enough together to prevent any traffic in the other lane from breaking it.
........With the Unsuspecting:
Drive the speed limit, or if you prefer a bit slower. Allow multiple people to pass you. Then pass them all and proceed to drive slowly again.

Road Block:
........With Friends:
Drive along side your buddy, preferably slow and late at night when people think the roads are open. If you wish to toy with impatient drivers slowly begin to pull ahead of your friend, but slow down before giving enough room to pass. You and your friends may also clump together to form larger and perhaps more elaborate road blocks. As long as you're all driving the speed limit, this should be perfectly legal, regardless of how much you back up traffic. But even if you are driving below the speed limit I doubt you'll get anything more than a light warning.
........With the Unsuspecting:
Find someone driving slow and stick with them at their side. Drive at the same speed, don't let anyone pass you. Once your fellow blocker turns off the road, proceed to find a new blocker.

Flood Gate:
As above but less assholey. This twist off the original is pretty much the same, except after you back traffic up to what you consider to be a sufficient amount, you and all your acomplices move into one lane and allow the poor trapped drivers behind you to go free.

Dumbass:
ALWAYS slow down to AT LEAST half the speed limit before passing through a green light.
If you ever get stopped at a light, when the light turns green wait AT LEAST a full 7 seconds before moving. NEVER turn right on red. Wait a full 10 seconds after fully and completely stopping at every stop sign. For an added bonus cut people off.
........With Friends: The first person to be called a dumbass, asshole, fucker (loud enough that they can hear it), or given the finger, by a nonparticipant wins. If you're headed to a destination, reach it first, without violating any of the above rules, and before anyone meets the above goals and you win.
........With the Unsuspecting:
Survive.

Classical Games

Chicken: Drive head on with someone. The first person to turn away loses. If no one turns the first person to die or fly through the windshield loses. If no one dies, the first person to be beaten and have their wallet taken loses.
With the Unsuspecting: You'll probably win, unless the unsuspecting happens to be a mirror. (ala Goldfinger)

Slug Bug:
Prerequisite: Must be retarded.
Whenever you see a VW Bug, announce "Slug Bug" and proceed to punch someone seated near you.

And that's all I could come up with.
I made these up. I did not steal them or rip them off or copy paste them from anywhere.
Comments welcome, praise welcome, players welcome, additional games welcome. Please share any thoughts. I can think of a good few more, but they aren't really all that legal.