Ada Class Library - an object orientated library for Ada. Text search and replace. Scripting (small tool programs). CGI scripts. Execution of external programs (incl. I/O redirection). Garbage Collection. Extendended Booch Components. CD-Recorder
This is an easy to use and extensible rules engine written in C# for .NET and Mono.
OpenBASIC is a modernized programming derived from the syntax of QBasic, QuickBasic, and FreeBASIC, yet has most (if not all) of the Object-Oriented features of C++, as well as the portability of that language coupled with Allegro for graphics routines.
A C++ plugin library
apugg is an object-oriented plugin loading library written in pure C++. It is designed to be usable across platforms, and adding support for new styles of plugins is done by simply inheriting a library class.
General purpose C++ utility library.
Libbu++ covers nearly everything you could need from a standard library, is cross platform, and open source. It has a number of cool and unique features and plays well with other libraries.
libthrowable is a small library to ease error handling and debugging in C++.