Brilliant math videos. Adam and Leora had a look at the first last night, and were impressed. (It takes a lot to impress two ten-year olds!) Thanks to Perdita Stevens for pointing me at these.
(Be sure to read the mouse-over.) Imagine the applications to cryptography! From XKCD, a webcomic of romance, sarcasm, math, and language, by Randall Munroe.