In their own words, SQL Fiddle is:
A tool for easy online testing and sharing of database problems and their solutions.
Some cool features included with this tool is being able to convert your SQL schema to a HTML formatted tree view, and the ability to preview your SQL output in an easy-to-read HTML table. If you use any form of SQL in your development environment, this can be a nifty tool to make testing much faster. Go check it out.