All you gotta do is name a song, yet has something in common with the above song. For example, if someone posted "The Trees", you could post "Colony of Birchmen" as they're both about trees, or if someone posted "I Think I'm Paranoid" you could post "Paranoid" as they have a word in common. Not limited to Rock Band songs. I'll start.
"Lithium" by Nirvana