This site is a glossary of statistical terms:

http://www.stat.berkeley.edu/users/stark/SticiGui/Text/gloss.htm

This site provides some practice with the mean:

http://www.robertniles.com/stats/mean.shtml

This site provides some practice with the median:

http://www.robertniles.com/stats/median.shtml

This interactive site calculates the mean, median, and mode for a data set.

http://www.calculatorsoup.com/calculators/statistics/mean-median-mode.php

Mean, median, and mode discussion.

http://www.shodor.org/interactivate/discussions/sd1.html

This is not a fancy site, but is interesting. It is more advanced than the textbook.

http://www.physics.csbsju.edu/stats/