Easy To Use Collection Classes For C And C++
The Memory Structures Library (MemSL) has implementations of structures (i.e. AVL trees, threaded trees, dynamic hashing, ...) that are extremely hard to find and far harder to implement.
JAST (Javascript Toolkit) is an Open Source cross-platform javascript framework that aid users to perform the most hard operations with few simple lines of code.
It's modularity allow the download only for the code required to the page.