Possibly one of my favorite instrumental songs ever conceived, Signal to Noise was written and sung by former 'Genesis' lead singer, Peter Gabriel. The score was used for the 2002 film, Gangs of New York. The music itself is a dramatic epic of sorts. Below is the song in both its normal, and purely instrumental forms, along with two music videos that use 'Signal'. Finally there's the scene from the Gangs of New York opening that showcases the music as well.
BE WARNED!!! Rated R violence and language is contained within the 3rd and 4th videos. If you don't want to see possibly disturbing images, don't watch 'em.
Here's the instrumental version of 'Signal' and by far my favorite.
Here's the normal version with Peter Gabriel providing vocals.
Here's a link to a Serenity video that features 'Signal'. Unfortunately embedding has been disabled for this video. http://www.youtube.com/watch?v=GX6IIiFfBXI
(Content Warning!) Granted, some portions of this video don't quite seem to match up, especially with the added end piece, but it's still pretty awesome.
(Content Warning!) Lastly, here's the fight scene at the beginning of Gangs of New York.