What references have shown up in numerous episodes?
Bottles of Yoo-hoo can be seen in a number of season two episodes, and there are constantly references to Buffy the Vampire Slayer and its star, Sarah Michelle Gellar. Corey Haim and Corey Feldman have also been mentioned repeatedly. (Completely random aside: As a total coincidence, I also created a Yoo-hoo site in 1995.