DTDDoc sounds interesting to me:
DTDDoc is designed to help document your DTDs efficiently. It is a straightforward extension of the Javadoc concept, and a not so straightforward implementation of some of the concepts solidified by Donald E. Knuth.
I haven’t seen this project before on freshmeat, but here’s what’s new in version 0.0.5:
New display of element/attribute parent, and a new ability to configure the title that is displayed on top of the index.
The example output comes with both html-ized javadoc-like human-readable output (too many dashes there), as well as a good old DTD that can be used to validate stuff. Quite a slick little project.