QuantLib is a cross-platform, quantitative finance C++ library for modeling, pricing, trading, and risk management in real-life. It is also wrapped as Python/Ruby/Scheme modules. Extensions for Excel, R, and Mathematica are available. Other such extensions are under consideration. QuantLib offers tools that are useful both for practical implementation and for advanced modeling. It features market conventions, yield curve models, solvers, PDEs, Monte Carlo (low-discrepancy included), exotic options, VAR, and so on.
BitTorrent is a tool for copying files from one machine to another. FTP punishes sites for being popular. Since all uploading is done from one place, a popular site needs big iron and big bandwidth. With BitTorrent, clients automatically mirror files they download, making the publisher's burden almost nothing.
Cheetah is a Python-powered template engine and code generator. It can be used as a standalone utility or it can be combined with other tools. Web developers are its principle user group, but it has many potential uses and is also being used to generate C++ game code, Java, SQL, form emails, and even Python code.
Kew is a simple, embeddable, container-based, object-oriented programming language. Many of its features are inspired by Smalltalk and Scheme. Its syntax is similar to Smalltalk's, but it has the compact and modular design of Scheme, along with proper closures and continuations. There is also powerful exception handling and write-your-own containers, which allow you to produce sandboxes.