Have a listen - this is a version by the The Sixteen (an incredible choir led by Harry Christophers), but the Tallis Scholars version is also beautiful:
There is a lovely bit of history that goes with this piece. The Pope at the time it was written thought it was so beautiful that he decreed that it was never allowed to be performed outside the walls of the Sistine Chapel in Rome. This was the case for over 100 years - the manuscript was never published - but it became legendary. Then, in 1700 a 14 year old lad (Wolfgang Amadeus Mozart) visited, heard it twice, then later wrote it down from memory. Apparently the original Miserere was simpler than the one we know today (as this little documentary shows) but still an impressive feat! After this, the Pope at the time congratulated Mozart on his musical talent, and the Miserere Mei was shared with the world.
Ah, I love choral music. Any favourites?