He's not that famous outside of hard core geek circles, but I am a huge fan of James Clark (not the SGI guy, but he's awesome too for different reasons), who wrote the Expat XML parser, Relax/NG XML schema language, and many other widely used standards and bodies of code that drive the Internet.
He has such clarity of thought and mastery of diverse languages (some of which he invented and implemented). He saw exactly what was wrong with XML Schemas, and addressed it practically and elegantly with TREX, which he and others refined through humble constructive collaboration into Relax/NG. And he's a big proponent and creator (not just a talker like ESR) of free open source software.
Here's a fascinating insightful DDJ interview of James Clark, "A Triumph of Simplicity: James Clark on Markup Languages and XML":
http://www.drdobbs.com/a-triumph-of-simplicity-james-clark-o...
>A Triumph of Simplicity: James Clark on Markup Languages and XML
>If you peek under the hood of high-profile open-source projects such as Mozilla, Apache, Perl, and Python, you'll find a little program called "expat" handling the XML parsing. If you've ever used the man command on your GNU/Linux distribution, then you've also used groff, the GNU version of the UNIX text formatting application, troff. If you've ever done any work with SGML, from generating documentation from DocBook to building your own SGML applications, you've undoubtedly come across sgmls, SP, and Jade.
>Whether you've heard of him or not (and mostly likely, you haven't), James Clark (below right) has made your life easier. In addition to authoring these and other widely used open-source tools (see http://www.jclark.com/ for a complete list), Clark served as the technical lead of the original W3C XML Working Group and as the editor of the XSLT and XPath recommendations. He recently founded Thai Open Source Software Center (http://www.thaiopensource.com/). His latest project is TREX, an XML schema language. Clark sat down with Eugene Eric Kim to discuss markup languages, the standardization process, and the importance of simplicity.
[...]
>DDJ: You're well known for writing very good reference implementations for SGML and XML Standards. How important is it for these reference implementations to be good implementations as opposed to just something that works?
>JC: Having a reference implementation that's too good can actually be a negative in some ways.
>DDJ: Why is that?
>JC: Well, because it discourages other people from implementing it. If you've got a standard, and you have only one real implementation, then you might as well not have bothered having a standard. You could have just defined the language by its implementation. The point of standards is that you can have multiple implementations, and they can all interoperate.
>You want to make the standard sufficiently easy to implement so that it's not so much work to do an implementation that people are discouraged by the presence of a good reference implementation from doing their own implementation.
>DDJ: Is that necessarily a bad thing? If you have a single implementation that's good enough so that other people don't feel like they have to write another implementation, don't you achieve what you want with a standard in that all implementations — in this case, there's only one of them — work the same?
>JC: For any standard that's really useful, there are different kinds of usage scenarios and different classes of users, and you can't have one implementation that fits all. Take SGML, for example. Sometimes you want a really heavy-weight implementation that does validation and provides lots of information about a document. Sometimes you'd like a much lighter weight implementation that just runs as fast as possible, doesn't validate, and doesn't provide much information about a document apart from elements and attributes and data. But because it's so much work to write an SGML parser, you end up having one SGML parser that supports everything needed for a huge variety of applications, which makes it a lot more complicated. It would be much nicer if you had one SGML parser that is perfect for this application, and another SGML parser that is perfect for this other application. To make that possible, the standard has to be sufficiently simple that it makes sense to have multiple implementations.