Randall's come out with a number of these.

Scale of the Universe, Radiation, IPv4 space, lakes & oceans, gravity wells, and Money all come to mind.

http://xkcd.com/195/

http://xkcd.com/980/

http://xkcd.com/482/

https://xkcd.com/1040/

http://xkcd.com/681/

(You can curse me later).