NOTE: This list was compiled in 2011, but it has been seeing a lot of hits lately so I thought it would be fun to repost with some new 2013 updates! Be sure to check the comments in the original post for more suggestions!
@jeannekalinow : Using & Handling Data @ Math Is Fun! Useful data and probability basics including qualitative or quantitative, discrete or continuous, data visualization, distribution curve and more! Credits: MathsIsFun.com is maintained by Rod Pierce DipCE BEng, with contributions from many others.