This is a java applet, which you need java to use. To download, go to http://java.sun.com/javase/downloads/index.jsp and download the latest 'Java Runtime Environment (JRE)', you do not need Java EE, Java SE, or JDK.
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For preflop computations, the overall percentage is the mean share of the pot that the pocket cards will win in a 10 player ring game with no betting or folding. The simulator runs by dealing 9 opponents and 5 board cards at random, finding the winner(s), and repeating.
Postflop: Unfortunately, the overall percentage has less meaning for posflop computations, because this program is not taking in factors like betting history and position. The simulator works by dealing 9 random opponents and removing the worst preflop hands using the rankings in this table, the number of folders is determined by: numFolders = 9 - remainingOpponents. The simulator then deals the five board cards, finds the winner(s) among remaining opponents, and repeats.
I am currently working on some poker ai, and there will be a signed applet / software comming out at some point that will read the cards / betting history into the program for you to give more advanced evaluations.