Run Estimator is a simple World of Warcraft addOn that works in the chatbox/console. It calculates the number of runs or kills a player has to make in order to reach a specified probability to get an item with a specific chance to drop. To use the addon type '/runs "dropchance" "desired probability"' without the ' ' (Example: /runs 32.15 90) This will output the line: 'RESULT: You need to make 6 runs to get an item with the dropchance 32.15% with 90% probability.'
This is a tool intended for use before you attempt to get an item, to get an idea of how many runs you will probably have to make, not to count "how likely am i to get [Judgment Hood] this time, because I have failed 17 times". (The answer to the latter question is obviously still the original dropchance.) The math is there and I intend to go into more detail documenting it in the addOn.