This applet computes probabilities for the negative binomial distribution $$X \sim NB(r, p)$$ where $$X = \mathrm{the\ number\ of\ failures\ before\ the}\ r^{th}\ \mathrm{success}$$
To compute a probability, select $P(X=x)$ from the drop-down box, enter a numeric $x$ value, and press "Enter" on your keyboard. The probability $P(X=x)$ will appear in the pink box. Select $P(X \leq x)$ from the drop-down box for a left-tail probability (this is the cdf).