Michelle Pfeiffer's famous rendition of "Makin' Whoopee" from The Fabulous Baker Boys.
UPDATE: Thanks for all the positive comments from like-minded filmgoers and Michelle Pfeiffer fans. If you haven't seen this movie you really should - it has a timeless, classic feel with a great script and superb acting. And if you rent the DVD you can see this scene in all of its full-screen, hi-def glory...way better than YouTube. Do it!