This week's book giveaway is in the OCMJEA forum. We're giving away four copies of OCM Java EE 6 Enterprise Architect Exam Guide and have Paul Allen & Joseph Bambara on-line! See this thread for details.
I think you need a freeware rather than an open source. Open source means the source code is available. I came across LiquidXml which promised alot of features and also has a free copy. Also, IDEs like eclipse and intellij idea have plugins for xml editing which are pretty neat.