There's no end to the amount of great songs about weed, but the first one I always think of is "Sweet Leaf" by Black Sabbath. Recent takes on "Sweet Leaf" by John Darnielle (in his terrific book on Master of Reality) and Jim DeRogatis (on Sound Opinions' excellent show on "unconventional love songs") are more profound than anything I could write, but the strongest case for "Sweet Leaf" is in the main riff.
Black Sabbath, "Sweet Leaf"
The unforgettable coughing intro and endlessly-copied riff are Tony Iommi's, but it's Ozzy who ensured that this would be metal's reigning 4/20 anthem. Check out some of his lyrics:
"You introduced me, to my mind
And left me wanting, you and your kind"
"I love you!
Oh you know it!"
"My life is free now, my life is clear
I love you sweet leaf, though you can't hear"
"You gave to me, a new belief
And soon the world, will love you sweet leaf"
Somehow, Black Sabbath are often perceived as humorless and foreboding. They may have been a musical alternative to the hippie bands of the late '60s, but when called to task, Sabbath could write a funnier, goofier ode to marijuana than anything this side of "Rainy Day Women #12 & 35."
"Sweet Leaf"'s contribution to the stoner music canon is undeniable. It was later reworked and renamed by the Butthole Surfers (as "Sweat Loaf") for Locust Abortion Technician. The result may be the greatest song that Gibby Haynes ever put his name on.
Butthole Surfers, "Sweat Loaf"
As if that weren't enough, the Beastie Boys introduced themselves to middle America by kicking off Licensed to Ill with a "Sweet Leaf" sample:
Beastie Boys, "Rhymin' & Stealin"
And listen closely to the riff at about 3:55 in the Red Hot Chili Peppers' signature hit:
Red Hot Chili Peppers, "Give it Away"
Not bad for some British kids singing about a plant.