MMC! And the Christmas music community on Livejournal. I know it's not a site technically, but it is a pretty amazing place, nonetheless. I usually make 1 Christmas mix per year. Last year, after finding and joining the comm, I made like 6! <3
My pal Rob's Christmas Yuleblog is certainly a favorite as are many other blogs in season.
Other non Christams places with way strong Christmas music content I like are NPR & Public Radio Exchange. This is where I dig up a lot of what I share in the Christmas audio archive thread each Rudolph Day.