It Don't Mean A Thing is almost the definition of a classic swing song. Here is a acapella version by the Swedish vocal group The Real Group from a concert at Stockholm Concert Hall in 2005.
I really like how the bass lines are sung and I'm also very fond of the way they layer their voices (starts at ~ 2:00 into the video clip).
What do you think of The Real Group singing It Don't Mean A Thing? Please comment.