SQLFiddle.com - Online playground for MySQL, PGSQL, SQLite and MSSQL

It's interesting that the whole stack behind the service is listed here:

http://www.sqlfiddle.com/about.html#source

Also the source code is available on Github:

https://github.com/jakefeasel/sqlfiddle

Disclaimer: I'm not the author and I'm not affiliated with the site in any way.