ditto on Voice Squad and King's Singers.
There is also a CD of English, French, and German songs recorded by Cambridge Singers, John Rutter, titled "A Cappella".
There is a men and boy's choir recording from St. Thomas Church called ... A Cappella from St. Thomas Church.
A seasonal one is "Carols from the Old and New Worlds", American, Irish, English, various choirs, Indiana University, Paul Hillier.
alice