XPathReader is/ was an implementation of a forward reading XML parser (built on XMLReader) which allowed you to register XPath queries for it to find (or at least a subset of XPath called Sequential XPath). This seems to be the perfect choice for easy access to elements of xml streams, or case where you just need to pull some information out of the start of a large xml document and therefore don't want to load the whole thing into memory.
There seemed to be a flurry of excitement about the open source implementation that one of the MS guys was releasing back in 2003/ 2004, eg:
http://donxml.com/allthingstechie/archive/2004/02/26/430.aspx
http://msdn.microsoft.com/en-us/library/ms950778.aspx
http://www.tkachenko.com/blog/archives/000472.html
But after that the trail seems to dry up. The references to where the implementation was hosted (http://workspaces.gotdotnet.com/xpathreader) no longer seem to work. Does anyone know what happened to it, and why the interested disappeared?