This is a compilation of many previous answers and adding of my own. Most of the credit to Paul Evans, Sleafar, charfellow, and question_asker (in no particular order).
The answer is:
You've probably seen me naked.
The Statue of David is a famous statue of King David in the nude. (from charfellow)
As an alphabet aficionado, I can make you laugh long into the night.
David Letterman, comedian and former late-show talk host. A letter man is an alphabet aficionado. (from question_asker)
As a make of Japanese vehicle, I can keep your surroundings clean and beautiful.
David Suzuki, environmental activist. Suzuki is a make of vehicle from Japan. (from question_asker)
As an execution apparatus, I make you laugh despite not maturing.
David Cross, comedian and actor on Arrested Development. A cross is an ancient method of execution, and not maturing = arrested development.
As a mixed-up romance, I lead tens of millions.
David Cameron, prime minister of the UK. Cameron is an anagram of romance. (from Paul Evans)
As a gardening tool, I provide weekend evening entertainment and provide guidelines for proposing.
David Spade, a comedian on Saturday Night Live and an actor in Rules of Engagement.
weekend evening entertainment = Saturday Night Live
guidelines for proposing = rules of engagement.
As a reddish-brown meadow, I dazzle you with tricks.
David Copperfield, illusionist. A reddish-brown meadow is a copper field. (from question_asker)
As a fighting knife, I make popular music and navigate a maze.
David Bowie, acclaimed singer, and actor in the movie Labyrinth. A Bowie knife is a popular knife originally made for fighting, and a maze is a labyrinth. (from Sleafar)
As a speech-impaired aquatic athlete, I am your friend.
David Schwimmer, actor on the TV show Friends. An aquatic athlete is a swimmer, which might be pronounced, "schwimmer" by someone with a speech impairment. (from charfellow)
As a renter, I am your doctor.
David Tennant, actor on the TV show Doctor Who. A renter is a tenant, and he plays the role of the doctor. (from charfellow)