a poster for it is on the wall in Leonard and Sheldon's apartment
Sheldon mentions playing "Klingon Boggle."
Leonard mentions his "Darth Vader shampoo."
The show is mentioned in the end credits "Chuck Lorre Productions, #182" card.
Penny uses it to describe Leonard and Sheldon
Sheldon wants to watch the Season 2 DVDs "with commentary"
Koothrappali wears a hat with "42" on it