JsonCpp provides an easy to use API for reading, writing and manipulating JSON values. It is written to be highly portable as long as your C++ compiler supports namespace and has "basic" STL support. JsonCpp has been around since 2005 and it is way past the time to make an official release.
The cvs repository has been migrated to subversion.
So it should now be possible to access from behind firewall. See subversion tab for detail.