Anime That Need a Second Season

We've all seen short anime that seemed a little too short. What are your favorites that ended too soon?

Mine are No Game No Life and Black Bullet. Both only have one season, with Black Bullet almost guaranteed to never get a second. There's still some hope for No Game No Life though, as some people who are close the the project have been caught admitting that a second season is in progress, but the information isn't confirmed.