A lightweight XML parser efficient for large files?

If you are using C, then you can use LibXML from the Gnome project. You can choose from DOM and SAX interfaces to your document, plus lots of additional features that have been developed over years. If you really want C++, then you can use libxml++, which is a C++ OO wrapper around LibXML.

The library has been proven again and again, is high performance, and can be compiled on almost any platform you can find.

I like ExPat
http://expat.sourceforge.net/

It is C based but there are several C++ wrappers around to help.

RapidXML is quite a fast parser for XML written in C++.