Yesterday I posted about a ping pong game that I had become addicted to and now I have a site with several different Flash games. You can find the list of games here and they are so simple that they are addicting. My favorite is Gravity which starts out pretty easy but it gets pretty hard.
One of the simplest game concepts would probably be Insanity. All you have to do is drop a ball in a hole and you get a point each time you drop it into the hole. I haven’t really found the point of this game because as the levels progress nothing really changes, but that is probably why this is called insanity. ;)
Note: You can find a game similar to Gravity here which also gets pretty challenging.