[c++] What XML parser should I use in C++?

I have XML documents that I need to parse and/or I need to build XML documents and write them to text (either files or memory). Since the C++ standard library does not have a library for this, what should I use?

Note: This is intended to be a definitive, C++-FAQ-style question for this. So yes, it is a duplicate of others. I did not simply appropriate those other questions because they tended to ask for something slightly more specific. This question is more generic.