A free cross-platform library of fast C++ routines for the plotting of up to 3-ranged data. It can export to bitmap and vector EPS/SVG files. There are window interfaces (GLUT/FLTK/Qt) and console tools. MathGL can be used from C/Fortran/Python/Octav/Lua
CoolReader is fast and small cross-platform XML/CSS based eBook reader for desktops and handheld devices. Supported formats: FB2, TXT, RTF, DOC, TCR, HTML, EPUB, CHM, PDB, MOBI. Platforms: Win32, Linux, Android. Ported on some eInk based devices.
QtXGame is an intuitive, extensible, modern cross-platform tool to construct and solve extensive and strategic games related to Game Theory. Its extensibility allows user to create and add his own games, solvers and matrix transformations.
hostingstar.com.au's open source software.
To be used primarily for dotnetnuke modules, a cross platform backup system, .net (dotnet) user controls, 3d plugins and many other bits and pieces.