A few movies I know of that are missing from the Wikipedia "List of drug films":
Jackie Brown - Bridget Fonda plays this stoner/surfer girl who spends well over half the movie smoking out Robert De Niro. Kinda odd that it's missing, since Quentin Tarantino directed it (and usually no internet list misses a chance to mention Tarantino films)
Salvador - James Woods play a Hunter-S-Thompson-esque journalist (with a Doctor Gonzo-like sidekick in Jim Belushi) in an early Oliver Stone film about the civil war in Salvador. It got a couple Oscar nominations, and is a great movie, very political though. According to the former ambassador to Salvador, it's pretty accurate too.
Bulworth - Not a huge pot film, but a some very funny drug-related humor. It's very political, and probably only enjoyable if you're liberal.
Annie Hall - Like "Bulworth", there's not a lot of pot scenes, but there are a few key scenes which revolve around it.
Serpico - Very funny scene where Al Pacino tries marijuana while working as a cop. If you're not a fan of the police, see this movie.
and while not a movie, the British TV series "Spaced" had a lot of hilarious pot smoking scenes. If you liked "Shawn of the Dead", you'll love "Spaced" (it features the same actors, writers, and director).