Big Lebowski.
Airplane.
Life of Brian.
Planes Trains and Automobiles.
Monty Python and the Holy Grail.
Definitely to all of these. Life Of Brian is one of the greatest comedies of all time.
As nands suggests Dr Strangelove is great although with an underlying message. Withnail and I or any of Kevin Smiths films are pretty humourous although you may wish to skip Jersey Girl.
This Is Spinal Tap is an awesome rock mockumentary, it does require a bit of attention to be paid though, you can't just dip in and out on a first viewing.
Shaun Of The Dead and to a lesser extent Hot Fuzz are good.
Van Wilder: Party Liason is quite entertaining in a light fluff sort of way - don't bother with the second one.