mxCrypto is an extension package that provides OO-style access to the cipher algorithms and hash functions included in OpenSSL/SSLeay, a very nifty cryptographic library originally written by Eric Young and now maintained by the OpenSSL team. Its main purpose is filling in the missing parts in Andrew Kuchling's pycrypt export package with high quality implementations. Due to the ITAR export restrictions on cryptographic software, Andrew's implementations are not legally downloadable from outside the US.
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.
ACDK is a development framework with a similar target of Microsoft's .NET or Sun's ONE platform, but it uses C++ as a core implementation language. It implements the standard library packages, including acdk::lang, acdk::lang::reflect, acdk::util, acdk::io, acdk::text (including regexpr), acdk::net, acdk::sql, acdk::xml, and more. Flexible allocator/garbage collection, threading, and Unicode are implemented in the core of ACDK. Extensions make C++ objects available for reflection, serialization, aspect-oriented class attributes, and [D]ynamic [M] ethod [I]nvocation. This DMI acts as an universal object oriented call interface to connect C++ with scripting languages (Java, Perl, Tcl, Python, Lisp, Visual Basic, and VBScript) and standard component technologies (CORBA and COM).
Xapian is a search engine library, scalable to collections containing hundreds of millions of documents. It's written in C++ with bindings for Perl, Python, PHP, Java, Tcl, C#, Ruby, and Lua. It is a highly adaptable toolkit that allows developers to easily add advanced indexing and search facilities to their own applications. It supports the Probabilistic Information Retrieval model and also a rich set of boolean query operators. Omega is a Web search application built upon the Xapian library. It can index a Web server's document tree (including HTML, PDF, OpenOffice, MS Word/Excel/Powerpoint/Works, WordPerfect, RTF, PS, etc.), or data exported from arbitrary sources (e.g. SQL databases).
Berkeley DB XML is a native XML database engine for use within your product. Made available as a C++ library with language bindings for Java, Perl, Python, PHP, and Tcl, it integrates directly into your application (it is not a standalone database server). It provides XQuery access into a database of document containers. XML documents are stored and indexed in their native format using Berkeley DB as the transactional database engine.
The Maximum Entropy Toolkit provides a set of tools and library for constructing maximum entropy (maxent) models in either Python or C++. It features conditional maximum entropy models, L-BFGS and GIS parameter estimation, Gaussian Prior smoothing, a C++ API, a Python extension module, a command line utility, and good documentation.