Working with DTD is still a common task for XML (/SGML) driven use-cases. Knowing this it is very amazing that there is no well know DTD visualization tool available supporting this task.
The good old "Near&Far Designer" is gone many years ago and the source is probably lost in the space of Open Text (the company bought Microstar Software Ltd in 1999). This tool is still in use by many organization having to deal with SGML DTDs (e.g. in the military or aircraft industry).
There are a few open source scripts out there which converting a DTD into HTML pages for documentation purpose which are available free of charge:
TreeVision (http://www.ovidius.com/meta/download/treevision.html) from German company Ovidius. The tool is available free of charge and provides a very sufficient way to analyze XML / SGML DTDs.
Convert to XML Schema alternatives
If you have to process the content of a DTD for specific use-cases like analyzing the model based on custom specific rules the easiest way is to convert the DTD to RELAX NG (XML syntax) or W3C Schema language. Both are based on XML and therefore can be processed using regular XML based tools.
The best tool to do support this is trang (http://www.thaiopensource.com/relaxng/trang.html source is hosted on http://code.google.com/p/jing-trang/) initially created by James Clark. Compared to commercial alternatives the result is very predictable and for many use cases as good as possible.
DTDs will still exists for many years just because of the many legacy applications created around them. The amount of support is limited but still exists....