Mule is an open source lightweight ESB that is quite simple to work with, but since configuring the flows is all xml-based it can be challenging to learn the ropes in the beginning.
After starting out with the default eclipse xml editor I noticed that it doesn't cope very well with the complex namespace structures that the configuration files require.
After trying out a couple editors I finally found the perfect match in the oXygen XML editor (http://www.oxygenxml.com/), whose autocompletion handles Mule ESB's namespaces perfectly. It also exists as an eclipse plugin, making it a good fit together with the Mule IDE (which is eclipse-based).
The downside? It's not free... So I'll be trying it out a little more to see whether the product warrants a purchase.
So, do you have an alternative xml editor that you can tip me about, and preferably a free/open source one? Give me your thoughts, especially if you've been using it in conjuction with Mule.
Over and out...