BottleRocket is a utility to allow the use of the X10 FireCracker wireless appliance control kit under Linux, FreeBSD, NetBSD and more; porting to other systems usually just requires changing the serial port device. It covers all of the major non-GUI functions of the X10 software for windows, plus many other features.
BRL-CAD is a powerful constructive solid geometry solid modeling system that includes an interactive geometry editor, ray-tracing support for rendering and geometric analysis, path-tracing for realistic image synthesis, network distributed framebuffer support, and image and signal-processing tools.
BZFlag is a 3D, multiplayer, tank battle zone, capture the flag game that pits players against each other in a networked environment. It runs on Windows 95/98/NT/2000, Linux, MacOS 10.x, Irix, Solaris, and others. An OpenGL accelerator is highly recommended, but it is playable with 3D in software.
C-XSC 2.0 is a C++ class library for eXtended Scientific Computing. It provides real, complex, interval, complex-interval, and multi-precision data types. There are also vector and matrix classes corresponding to these scalar types. The interval data types are very useful for verified numerical computations.
Calc is arbitrary precision arithmetic system that uses a C-like language. It's useful as a calculator, an algorithm prototype, and as a mathematical research tool. More importantly, calc provides a machine-independent means of computation. Calc comes with a rich set of builtin mathematical and programmatic functions.