Not including the songs everybody heard about, songs I've found that I never heard of (often never heard of the artist/band either) before I found them in RBN:
"Stegosaurus Song" by Eric Harm: Easy song but fun, especially to sing. Sounds like something The Grateful Dead or Phish would have come up with.
"The Most Beautiful Girl (In the Room)" by Flight of the Conchords: Hilarious lyrics.
"Too Hot to Handle" by Carl Douglas: I knew of Carl Douglas solely for "Kung Fu Fighting," thought he was pretty much a one hit wonder, but this song is awesome. Rock Band could use more funk like this.
"VP of Booty Reports" and "Grumpytown" by Speck: Never heard of them before hearing "Conventional Lover" in RB2. Both funny songs on vocals.
"Upstream" by Cory Wong: First guitar only song. Fun to play.
"Moonshine Hollar" by Zach Wilson: Hilarious vocals, and fun to play on any instrument.