This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.