libxml2 is similar to libxml. It is required and supported by the gnome project. It allows applications to access HTML and XML documents without the need to program their own systems to read the files.
If you can help it (like using a Gnome 1.x library that still links it), DO NOT USE libxml1.
Some other things to point out is that it has internal support for fetching XML docs over HTTP and FTP too. It also completely supports ?XPath, ?XPointer, and ?XInclude, and support loading progressively or allowing it to doing a block type loading.
See more at: http://www.xmlsoft.org/
This library is released under the MIT License.