The first one is the one most people use, written by dethgrind from 2+2:
http://www.bol.ucla.edu/~sharnett/ICM/ICM.html
This one has a good explanation of ICM, it's run in javascript so it's quite fast, and there's a lot of nice additional material.
I've also written one myself:
http://www.chillin411.com/icmcalc.php
While I don't have much of the math or discussion, mine works for the multi-table SnGs as well as the single table ones, and I think mine's prettier
Just thought I'd share the two, so we both have the links in a central location, and those who are looking to learn ICM can start fiddling around with them.

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