Graphpad

http://www.graphpad.com/quickcalcs/index.cfm

Nice web-based calculators and statistical tools:

Categorical data
Fisher's, Chi square, McNemar's, Sign test, CI of proportion, NNT (number needed to treat).

Continuous data
Descriptive statistics, detect outlier, t test, CI of mean/difference/ratio/SD, post tests.

Statistical distributions and interpreting P values
Calculate P from t, z, r, F or chi-square, or vice-versa. View Binomial, Poisson or Gaussian distribution. Correct a P value for multiple comparisons and Bayes.

Random numbers
Assign subjects to groups, simulate data.

Chemical and radiochemical data
Create and convert molar solutions, convert moles & grams, radioactivity calculations.