When Leonard (Johnny Galecki) gives Penny (Kaley Cuoco) the number for Sheldon (Jim Parsons)'s mother, he paraphrases Harry S. Truman's secretary of war, Henry L. Stimson, who justified the use of the Atomic Bomb by stating that it will shorten the war by five years and save millions of lives.
The Klingon Dictionary that the guys are using is real. It was published by Simon and Schuster in 1992.
The two models who open the door for Raj (Kunal Nayyar) and Howard (Simon Helberg) are Lio Tipton and Samantha Potter, who were actual contestants on America's Next Top Model (2003). In their cycle, they placed third and second, respectively.
When Howard (Simon Helberg) sarcastically chants "One of us, one of us" after Leonard says "You're officially one of us!", this is an homage to the 1932 film Freaks (1932) where deformed humans are joyfully chanting for the female protagonist Cleopatra (to whom Penny is being compared) "Gooble gobble! We accept her! One of us! One of us!"
The correct spelling of "Qochbe'" (klingon for "To agree") is with a "Qo", not a "Ko".