migrated from stackoverflow.com Sep 3 '11 at 17:41
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Since Mar 2011, they are using Dapper.
Here's the introduction post for Dapper by the developer Sam.
Below is the original accepted answer which is out of date.
http://blog.stackoverflow.com/2008/09/what-was-stack-overflow-built-with/This will go into more detail concerning the stack that StackOverflow uses (including it's ORM).