It is Echoes by Pink Floyd. It displays their talent and range as a progressive band and its extremely haunting and beautiful.
Here's a link to their exquisite live rendition in Pompeii (The song doesn't really start until the 3:20 mark, so either be patient or fast forward.): [ame]http://www.youtube.com/watch?v=Hurn3-Y2b1g[/ame]
It's a song that lasts over 20 minutes, so here's parts two, three, and four.
[ame]http://www.youtube.com/watch?v=6y1bTyOMqqU[/ame]
[ame]http://www.youtube.com/watch?v=HqmJKXNW6ow[/ame]
[ame]http://www.youtube.com/watch?v=rLjXgRo2WGs[/ame]