We’ve had the longest, now it’s time for the shortest.
The shortest track I have is from Mozart’s The Magic Flute. It consists of the spoken words “Ist es denn wirklichkeit, was ich sah?” or “Is it really what I saw?”
The track is 3 seconds long.
Napalm Death - You Suffer (1.316 seconds)
The shortest track I’ve heard that could still be considered a “song” by me is Weinerschnitzel by the Descendants at around 10 seconds. They also had a couple shorter tracks called some variant on “All” but I wouldn’t consider those songs per se.
Snowboarder Bo’s example may be impossible to beat.
They Might Be Giants’ album Apollo 18 contains a medley of very short songs called “Fingertips,” each of which is its own track on the CD. (It was written to take advantage of the then-somewhat-novel shuffle play feature on CD players.) The shortest songs (“I heard a sound” and “Who’s knocking”) are 4 seconds each.
“I Say Fuck” by the Supersuckers is really short, but it’s long compared to some of the offerings here. Maybe it qualifies as the shortest song with 2 verses and a drawn out ending. (At work, can’t currently get a youtube link for a song with that title 
)
Here you go:
(My work has a pretty liberal internet use policy. I mean, I wrote it...)The Beatles Her Majesty at 23 seconds.
Stormtroopers Of Death’s brilliant debut album Speak English Or Die (1985) featured 3 songs under 10 seconds:
S.O.D. - Hey Gordy! (:08)
S.O.D. - Diamonds and Rust (Extended Version) (:07) (yes, this is their version of the Joan Baez song; inspired no doubt by the Judas Priest cover on Sin After Sin)
S.O.D. - The Ballad of Jimi Hendrix (:06)
For their 1999 follow-up Bigger Than The Devil they kept that last joke alive with 2 songs:
S.O.D. - Ballad of Michael H. (:12)
S.O.D. - Ballad of Phil H. (:08)
“There’s a Riot Goin’ On” by Sly and the Family Stone, listed on the (original album) label as 0:00!
I just searched my music folders for short tracks, and this was the shortest I came up with, but it’s only 4 seconds of silence, so I thought it was a badly ripped file. I’ve ripped all my CDs and have never noticed this oddity when listening to the CD. The other short track I found is the last track on Kyuss’ “Blues For The Red Sun” called “Yeah”, a four second track of someone saying, well, “Yeah”.
ETA: the shortest track I found with actual music is “No Money, No Honey (intro)” from “Stereopathetic Soulmanure” by Beck, 6 seconds.
Queen’s posthumous album Made In Heaven also includes a four-second track “Yeah.”
Off the top of my head, I’d say one of Blink 182’s joke tracks, Build this Pool, at 15 seconds.
ETA, Snowborder Bo’s knocking it out of the park!
Well, in this case I guess that Kyuss paid tribute to Freddie.
Government Issue - Stepping Stone (listed at 7 seconds, but there’s about 1.5 combined seconds of silence at the beginning and end of the track, so more like 5).
Janet Jackson’s album janet. includes a dozen tracks that are under 15 seconds each, with the shortest at 4 seconds long.
My first thought was the “filler” tracks on CDs to space out the hidden tracks. E.g., on Cracker’s Kerosene Hat. But those are apparently 4-6 seconds long. Surely there must be some really short such filler out there.
And if you don’t think silence isn’t a song, tell that to John Cage. Well, or his ghost or whatever.
Another four-second track: The Flower Kings, “Ikea by Night”, Flower Power.
Just for posterity: remember the Broken EP by Nine Inch Nails? On the CD: tracks 1-6 were songs, then tracks 7-97 were each 1 second long silent tracks, and tracks 98 & 99 were two more “hidden” songs. Maybe other artists employed this trick too, but this is the one i remember/am aware of.
Not that it’s going to beat anything already here, but I just want to add “Harvey the Wonder Hamster” by Weird Al Yankovic from the *Alapalooza *album clocking in at 21 seconds.
“Ban the Game” by Men Without Hats. There’s a longer version, but the one on the Rhythm of Youth album was 48 seconds.