{Editor's Note: This is a semi-regular series of high-water mark performances that spans movies, television, books and life its ownself.}
This isn't Matt Damon's Up On The Mountain for leading roles, but if you're discussing glorified cameo appearances then it's unlikely he'll ever top his turn as a shaven-headed lead singer for a rock band in "Eurotrip."
The film is a decently funny low-brow rental, but Damon ups the ante with his appearance. 
Damon is the catalyst for letting the main character, Scotty, know that his girlfriend, Fiona, has been cheating on him with virtually everybody in town. This occurs at a party where Damon's band is playing. He takes the microphone and screams:
"Hey, listen up, everybody. Gotta a little special thing I'd like to do tonight. Going to play a little song for you about the nastiest, freakiest little sex puppet I know . . . Fiona."
What a great intro, and Damon's look, which is so far removed from his typical persona, fits perfectly with the character and lyrics of the song he launches into to kick off the party:
" Scotty doesn't know
That Fiona and me
Do it in my van every Sunday
She tells him she's in church
But she doesn't go
Still she's on her knees
And Scotty doesn't know
Oh, Scotty doesn't know
So don't tell Scotty
Scotty doesn't know
What a great way to hear that your girlfriend has the morals of a goat, eh?
But nobody could have delivered it as well as Damon as his cameo was unexpected and hilarious - both of which are necessary for a token appearance to work well and he nailed it as the best little part of his career.
-BDS
That does not even LOOK like Matt Damon! hahaa I have to see this now. He cracked me up on "Stuck on You" as well. I think people underestimate his comedic talent.
I still think Matt's best performance was in "Team America: World Police."
Jlee - His look just makes it all the better, and he does have a good sense of humor (also see cameo in "Jay and Silent Bob Strike Back for evidence."
Wendy - That is another good one too. Funny as hell.
Ok, it's added to Netflix. I would never see this movie without such a high recommendation.
The movie itself is only OK, but if you go in with low expectations and get a little sideways before seeing it, then it's probably worth going in the acct. Probably.