SimpleXlsxWriter
C++ library for creating XLSX files for MS Excel 2007 and above.
... is written in C++ with using STL functionality and based on the ZIP library (included), which has a free license:
http://www.codeproject.com/Articles/7530/Zip-Utils-clean-elegant-simple-C-Win32
This library is distributed under the terms of the zlib license:
http://www.zlib.net/zlib_license.html