Software Recommendations

Does anyone know of a good program for coding in XML (something that does MathML would be good too)? I already have DTDs but I don't have something that will run them against text or help me visually organize the XML elements as I'm writing them or anything like that. The stuff we have at work is not built for this kind of thing.

By the way, this isn't limited to free software, although if it isn't free I would like to try a demo first. It's for work, so if it fits the needs of the project, we'll pay for it.

Any suggestions?