Libcwd is a full-featured and well-documented library that assists C++ developers with debugging their applications. It includes support for ostream-based debug output, custom debug channels and devices, memory allocation debugging, run-time sourcefile:linenumber information, and demangled type names of variables. It is thread-safe.
The Qt XInput Extension is a C library adding support for XInput devices (like tablets or joysticks) to Qt-based X11 applications. QXi supports devices with up to six valuator axes in absolute mode, up to 32 buttons, and any number of keys. It is built using autoconf and libtool, and comes with a simple example application.