Ahhh, isn't it beautiful? It goes on for about fifteen minutes, too, so it's perfect chillout stuff (try it as a lullaby, if you've got little ones).
For anyone who can't be bothered to follow 4getmenot's link, it's Fantasia on a Theme by Thomas Tallis, by Vaughan Wiliams.
Long winded name, eh? Apparently it's because, in the 1910s, Ralph Vaughan Williams elaborated on a tune written by Tom Tallis in the 1500s. Posh people call it the Vaughan Williams' Tallis Theme, which is slightly shorter but much more pretentious.
If you like it, try Vaughan Williams' smash hit The Lark Ascending.