aha! You are speaking of "Blue", which is written and by Yoko Kanno, the musical genius who came up with "Tank!", the show's openning theme. It's a really nice song, an excellent download. If you want to see a really good Anime Music Video titled "Life is but a dream", featuring the song with Cowboy Bebop footage, click here. It's a great music video for a great song. But don't download it unless you don't mind spoilers or have seen the last episode (which you have).