Evolvotron is an interactive "generative art" application to evolve images/textures/patterns through an iterative process of random mutation and user-selection driven evolution. If you like lava lamps, and never got tired of the Mandelbrot set, this could be the software for you. It's implemented using Qt, and is multithreaded.
NNDB is a C++ library that provides in-memory data storage and retrieval using syntax that is intended to feel natural for C++/STL, but which also might feel slightly like SQL if you squint and pound the top of your head with a seminal programming textbook. Columns types are fixed at compile-time and checked by the compiler.
Metacza is a language and its compiler which produces output in the C++ Meta Template Language. Its concise syntax makes Meta Template Programming much easier. Metacza allows you to write programs using boost::mpl, and to produce C++ header files that can be used on their own. Programming without boost::mpl is also supported. Metacza is a higher order language with closures, let, and lambda.
LIME is a dynamic programming language with a LISP-like syntax. It features partial function application, eager and lazy evaluation, call-by-value and call-by-reference, macros, reading/writing LIME code as data, a standard library including infinite streams, and an interactive mode with auto-indentation.
The YB.ORM library aims to simplify writing C++ code that has to deal with SQL databases. The goal is to provide a convenient interface like SQLAlchemy (Python) or Hibernate (Java). The library itself is cross-platform and supports a variety of SQL dialects: SQLite3, MySQL, Postgres, Oracle, and Firebird. Integration with Boost, Qt4, and wxWidgets is built-in. In a typical usage scenario, you would describe your database schema and table relationships in a simple XML-based format, generate SQL code to populate database schema with tables, generate C++ classes, add application-specific logic to the classes, and use these classes in cooperation with the Session object to query objects from the database, create new or modify/delete existing objects, or link and unlink objects using relations. Simple serialization to XML is supported along with connection pooling.