Mordecai and Rigby doing a Training Montage on an arcade game while "Hangin' Tough" plays over the sequence. Seriously, the writers seem to love dated 80s music. It's pretty awesome that the producers are willing to pay for all this music, since the licensing fees must be huge.
The score piece that plays during the audit in "Don". It almost sounds like it belongs in a theatrical production.
"Hey Man, Nice Shot" by Filter playing during the climax of "Slam Dunk" (even though that song isn't about basketball playing; it's about the suicide of a corrupt politician that was broadcast on live television).
Mordecai has a training montage to "Holdin' Out For A Hero" so he can eat a 12-egg omelette in under an hour in "Eggsellent".
Dies Irae plays when Skips beats Rigby in arm wrestling and kills him.