Coheed and Cambira

Nimmlies, high vocals, power-chords, and sci-fi lyrics. Best of all, its free. It's Coheed and Cambria and they have a purevolume page, and here are the direct download links:

Welcome Home Ã¢â¬" Good Apollo, I'm Burning Star IV, Volume One: From Fear Through The Eyes of Madness
The Crowing Ã¢â¬" In Keeping Secrets of Silent Earth: 3
The Suffering Ã¢â¬" Good Apollo, I'm Burning Star IV, Volume One: From Fear Through The Eyes of Madness

You'll probably need to rename these to .mp3 for them to play.

update: fixed the album for The suffering.