Osu

Osu, officially known as "osu!" is a freeware rhythm game made by Dean "peppy" Herbert, based on the gameplay of the Ouendan and Elite Beat Agents series of games for the DS.

Game levels, called beatmaps, are created and submitted by users via a Level Editor, refined through community modding and ultimately approved by members of the Modding Assistance and Beatmap Appreciation Teams.

Besides the Ouendan/EBA gameplay, osu! includes two additional game modes: one based on the Taiko no Tatsujin series and another revolving around catching fruit. Players can compete in all three modes in online multiplayer matches.

Its website can be found here and the game can be downloaded here.

This game provides examples of:

 * Anime Theme Songs: Around 1/3 of all the ranked maps are these.
 * April Fools Day: The jokes each year revolve around changes in the game's genre ("Touhosu", a Touhou/osu! hybrid), gameplay ("osu!core", where the songs in every beatmap had their pitch and speed increased), staff (unexpected demotions or promotions) and beatmaps (pretending to approve and praise an extremely poorly made beatmap).
 * Call a Hit Point A Smeerp: The currency of this game is called "kudosu", and is earned through modding.
 * Crossover: The beatmap for Queen's Don't Stop Me Now is based on the plot of a Fanfic which combines Phoenix Wright with Elite Beat Agents. It's as awesome as it sounds.
 * Determinator: A player can become this if they're trying to pass, get an FC (Full Combo), or enter the Top 40 on a beatmap, especially if the beatmap is pretty difficult.
 * Fan Nickname: peppy is occasionally called pepsi. Also, osu! users can add their own nicknames to their "highlights" list.
 * Fun With Acronyms: MAT (Modding Assistance Team), BAT (Beatmap Appreciation Team) and, until its merging into the BAT, GMT (Global Moderator Team).
 * Grade Inflation: As in Ouendan and EBA, S comes after A, but a perfect performance in osu! earns the player an SS rank. An SS achieved under the Hidden and/or Flashlight mods has a separate sprite (alternate colouration in the official skin) and commonly referred to as "SSH".
 * Idiosyncratic Difficulty Levels: Various beatmaps do this.
 * Harder Than Hard: Many beatmaps have an Insane difficulty, and some even have a difficulty above Insane.
 * Interface Screw: The Hidden and Flashlight mods.
 * Level Editor
 * Mascot: Pippi.
 * Marathon Level: There are a handful of "marathon" beatmaps, commonly ranging between 10-15 minutes in length. The longest, set to paraparaMAX I, is over 36 minutes long.
 * Nintendo Hard: Thought Elite Beat Agents and Osu! Tatakae! Ouendan were hard? You haven't seen anything yet.
 * This especially applies for the beatmaps with >=4.95 SD (Star Difficulty). And approval beatmaps with crazy streams/jumps. An example is this beatmap, based from "Can't Defeat Airman".
 * Positive Discrimination: One reason is that Cookiezi, one of the top osu! players, is Korean.
 * Rage Quit: A common practice to avoid submitting and saving low accuracy scores.
 * Rhythm Game
 * Self Imposed Challenge: Using one or more of the difficulty increasing mods and/or going for SS (perfect) runs.
 * Stylistic Suck: The song Aha? is awful. Yet so very good. Notable for being the lowest-rated beatmap on the site.
 * The Jarto series is based on this trope.
 * Don't forget Spinsmith!
 * ztrout is like Aha?, except it came first.
 * Super Deformed: Miniature versions of Pippi, Ryuuta, and J can be found on the homepage and Catch The Beat.