From The Panty Pinata Polarization:
Leonard, after Penny is banished: "Just so I know, would you be open to taking his class? You can do it online."
Penny: Leonard, remember when I said it was on? Well, now it’s junior rodeo on!
Leonard: Oh, not junior rodeo. (A lot of the funny was Leonard's inflection.)
From The Euclid Alternative:
Leonard: Aw, the pet store.
I also like the running joke of Sheldon's wifi passwords.