From Phil Plait at Bad Astronomy, a list of ten things you might not know about black holes. Some of this I knew, but this one is incredible:
If you were to rope off the solar system out past Neptune, enclose it in a giant sphere, and fill it with air, it would be a black hole!
See also this recent tweet from physicist Brian Greene:
Remove all the space within the atoms making up the human body, and every person that's ever lived would fit inside a baseball.
(via @daveg & @rosecrans)