There's quite a few jabs at William Shatner's iconic Captain Kirk role, such as when Denny is given a cellphone by one of his many girlfriends and it makes the Star Trek communicator beeping sound whenever he flips it open.
Also, when describing a parasitic water bug,
Alan: They call them cling-ons.
Denny: Did you say Klingons!?
The third season episode "Son of the Defender" used footage from the 1950's Defender TV movie featuring Shatner to present Denny as a young attorney clashing with his father. It was surprisingly emotional.
In one episode, Valerie Bertinelli appeared as a guest character, and her appearance was heralded with the theme from One Day At A Time, the show that made her a star.