I'm very fond of A Tapestry of Carols by Maddy Prior and the Carnival Band -- many of their carols were new to me, but once you know them they're eminently singable. Here's their version of The Holly and The Ivy (http://www.youtube.com/watch?v=rcAjCm8L9Ws), for instance.

I also love listening to A Renaissance Christmas (http://www.amazon.com/Renaissance-Christmas-Cohen-Boston-Camerata/dp/B000005IXX) by the Boston Camerata... you can listen to some samples on that page if you like. Not sure if they fit your definition of "singable", but I sing along with them anyway. :D