Wordle

http://www.wordle.net/

Wordle is a toy for generating "word clouds" from text that you
provide. The clouds give greater prominence to words that appear more
frequently in the source text. You can tweak your clouds with
different fonts, layouts, and color schemes. The images you create
with Wordle are yours to use however you like. You can print them out,
or save them to the Wordle gallery to share with your friends.