Synth is a C++ template framework - a set of components that can be mixed and matched to build the right functionality; furthermore, components are loosely-coupled, designed to be both extensible and replaceable. Synth blurs the line between compile-time and runtime, and it does so by blending the static C++ type system, the dynamic values that need to be manipulated and formatted, including those from other languages, and the templates to do so. The name is an allusion to this synthesis process, which combines values to generate new ones (streams, files, strings, numbers, etc.)
QMentat helps you practice mental arithmetic. It uses arbitrary-length fixed point arithmetic, and can handle any size numbers, only limited by the size of your screen (and in the case of division with an integer result, the speed at which the number can be factored). It is also quite configurable, allowing you to tailor the questions to your needs.