StepMania

StepMania is an open source and cross-platform rhythm video game and engine. It was originally developed as a simulator of Konami's popular arcade game series Dance Dance Revolution, and has since evolved into an extensible rhythm game engine capable of supporting a wide variety of rhythm-based game types. Released under the MIT License, StepMania is free software.

The primary game type features the following game play: as arrows scroll upwards on the screen, they meet a normally stationary set of target arrows. When they do, the player presses the corresponding arrows on his or her keyboard or dance mat. The moving arrows meet the targets based on the beat of the song. Stepmania strongly utilizes a player's sense of rhythm in its game play. The game is scored based upon how accurately the player can trigger the arrows in time to the beat of the song. The player's efforts are awarded by letter grades and a number score that tell him/her how well they have done. An award of AAAA (quadruple A) is the highest possible award available on a standard installation and indicates that a player has triggered all arrows with "marvelous" timing (within 0.0225 seconds under official settings) and avoided all mines and completed all hold arrows. However, a patch is available that provides "ridiculous" timing (within a window of 0.01125 seconds, or half the timing of marvelous) and a top grade of AAAAA (quintuple A). An E indicates failure for a player to survive the length of the song without completely draining his/her life bar. Default scoring and grading for Stepmania is almost identical to scoring in Dance Dance Revolution; however, timing and scoring settings can easily be changed.

Source: Wikipedia, "Stepmania", available under the CC-BY-SA License.