This stuff is sorted by programming language -- not a perfect choice, perhaps, but better than total chaos.

FileOpener: Small C++ class to facilitate frequently used file initializations
Numerical C++ libraries: Complex arithmetics, linear algebra, splines,...

Scatter: Small, fast program for computing acoustic scattering from a smooth periodic surface. With only 30 KB executable, incorporates a smart algorithm for solving boundary integral equations.
Pas4PS: Object Pascal (Borland) library for programming PostScript pictures. Utilities for polygons, arcs, arrows, dashing etc. Main target: figures for scientific papers. The library can be used to create custom-tailored graphs of functions, Poincare sections, and similar calculation-based graphics, which sometimes looks better than similar graphics generated by Maple, Mathematica, etc, or simply can not be effectively produced by brandname packages.
Runge-Kutta-Fehlberg integrator: (RKF of order 4-5) for ordinary differential equations with an abstract callback handler which can fetch integration data upon request at user-defined moments.