K-ON!

K-On! is a manga series that began in 2007 and is still ongoing. It is about girls who form a light music club and perform as a band.

In 2009 (April-June) there was a 13-episode anime adaptation of the same name.

In January 2010 there was an OVA called "Live House!".

A 2nd season (sequel) to the anime aired April-September 2010, this time accumulating 26 episodes. It's title was slightly different, indicated by an added exclamation mark.

A March 2011 OVA called "Plan!!" also aired. Then there was an anime film in December 2011.

This anime has a fan wiki dedicated to it at w:c:k-on.