Starting with Mule 3.3, the Mule ESB open source community built-in support for a new expression language for processing message flows in a very powerful and elegant way.
It is very powerful and based on the MVEL language (note you can also use JavaScript, Groovy, Python and Ruby). MEL provides a very easy way to access the current message in a flow, write conditional filters and perform content based routing. The MEL Cheat Sheet, gives you an overview of the commands you can use. -- David Dossot
Now there's an official MuleSoft cheat sheet for the Mule Expression Language. You ought to check it out if you're a Mule ESB user, or looking for an ESB based on the ease of their processing flows.
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