I rescind all previous votes. Cheech and Chong wins.
Joe
slight hijack
Jonathan Coulton , the songwriter for “Still Alive,” makes a lot of really cool off-color songs that you can listen to on his website.
Also, a very talented machinima artist by the name of Mike Spiff Booth put a whole bunch of his songs to video, utilizing models and settings from World of Warcraft.
A few of my favorites:
Code Monkey
The Future Soon
I Feel Fantastic
re: Your Brains
Kind of reminds me of They Might Be Giants.
“Think About It” by Flight of the Conchords.
“Drink Till I Die” by Bad News:
Gimme another drink, Mister Bartender
If you don’t I’m gonna stick your dick in a blender.
'Cos I, oh I… I’m gonna drink till I die!
Vodka makes me happy, lager makes me mean,
I’m a heavy metal drinking driving alcohol machine.
Thanks, I came in to post that exact clip: Big Bottom live from the Live Earth concert, with members of Metallica, Beastie Boys, and several other bands, all playing bass. Beautiful!
No, the Folksmen did Never Did No Wandering. The New Main Street Singers did my personal favorite, Potato’s in the Paddy Wagon:
In fact, just the introductions are so funny I almost busted a gut listening:
On bass: X with Madonna, On bass: Y with Madonna, eventually everyone does Madonna.
On bass drum: Skippy Scuffleton–pray for him!
Amongst other gems.
“Hard Out Here For a Pimp” from Hustle & Flow.
Also second “Fever Dog” from Almost Famous.
Being more of a fan of the “oldies”, I prefer Cups and Cakes by The Thamesmen. 
And no love for Bloat On by The Bloaters (Cheech and Chong)?
Then you’ll love this!
“Ballroom Blitz” by Crucial Taunt.
Wayne is right, Cassandra really can wail.
I’ll confess that “Music and Lyrics” is a guilty favorite – “Pop Goes My Heart,” “Dance With Me Tonight,” "“Meaningless Kiss,” and “Way Back Into Love,” are all songs I like. Yes, I bought the soundtrack.
In the movie, the concert was live, and the NMSS hijacked things by performing their version of “Wanderin’” to the chagrin of the Folksmen. The transsexual bass player suggested “Give the audience a choice” and perform their version anyway, but he was overruled. So the Folksmen performed a medly of their hit, and then “Barnyard Symphony”, and were stalling while Mitch returned from wandering the streets.
Yes, I own the DVD. The concert end-to-end is one of the special features, and it is as funny as the rest of the film.
Regards,
Shodan
Heck, I got the whole soundtrack for That Thing You Do! Lots of great stuff.
I wish Corn Wine had made it onto the Mighty Wind soundtrack CD. And the Folksmen’s version of “Start Me Up” caught me completely by surprise . . . I thought the words sounded familiar, but it took me a verse or two to place them. “You make a dead man commmmmmmme . . .” 
Punditlisa already mentioned Eddie and the Cruisers.
I love the whole soundtrack but I’ll pick the standard Mustang Sally by the Commitments.
“Join the Professionals” from Ladies and Gentlemen, The Fabulous Stains. Also Diane Lane’s rant from early in the film when she screams about being perfect but not putting out. Why is that film not on DVD?!
Oops, having gone through a bunch of the videos on YouTube I would like to change my choice to Treat Her Right by The Commitments.
Wasn’t that the one written in D minor, the saddest of all chords?
Keys, that is.
David Brent, solo artist, singing Free Love Freeway. Man, thats one catchy tune. He also has a political reggae song called Equality Street.
If you like the solo version, then you will love the “studio version” as one of the extras (no pun intended 
on the “The Office” Specials DVD.
There’s also a music video of Brent doing a ballad that’s hilarious on the specials DVD (David Brent does Michael Bolton with all the cliche ballad music video clips).