Cookie Clicker is a free online game developed by French programmer Julien "Orteil" Thiennot. The objective of the game is to click on the giant cookie on the screen as much as possible in order to gain virtual currency, called "cookies". The player can then use the cookies to purchase upgrades and various buildings, which enable the player to generate even more cookies. As the game progresses, the player can unlock new buildings, upgrades, and other features, such as the ability to hire helpers and purchase additional upgrades. Additionally, the game includes various mini-games and achievements. The goal of the game is to eventually reach the highest level of cookies, called a "Cookie God".
XKCD, although it is the most difficult to play.