Brainchild of Joss Whedon, "Dr. Horrible's Sing-Along Blog" is a web-only three-act musical production thats currently available on Hulu.com. It features Neil Patrick Harris as "Dr. Horrible" who is trying to earn his way into the Evil League of Evil, led by super-villain "Bad Horse", and Nathan Fillion (from "Firefox") as "Captain Hammer", a self-centered hero. Obviously its got some comedic elements.
The show features some great counterpoint songs, and I've embedded the beginning of act two that has one of these songs. The girl singing is "Penny", a girl that Dr. Horrible has a crush on, that Captain Hammer has just started dating and helping with a project to help the homeless.
Also, if you mouse of the bottom of the video panel you'll see a pop-up menu that include a button marked "CC". Go there and you can turn on closed captioning, allowing you to "Sing-Along".
The full feature is available on Hulu, and I recommend it with the caveat that there are a few lines of PG-13 rated dialogue.