Agar
Agar is a powerful open-source, cross-platform toolkit for graphical applications in C / C++ (with bindings to other languages). Designed for ease of integration, Agar follows the philosophy of building the GUI around the application, and not the other way around. Agar applications are easily ported to any platform (including embedded platforms without OS), and Agar itself is free of dependencies. When compiled with optional threads support, Agar's entire documented API is thread-safe.