Identix - BioEngine SDK
Toolkits and sample code provide verification (one-to-one) and identification (one-to-many) matching capabilities. Has image acquisition library, quality control library, processing library, search library, and demo applications.
http://identix.com/products/pro_sdks_bioengine.html

Intel Math Kernel Library
Intel® Math Kernel Library 10.1 - Overview
http://www.intel.com/software/products/mkl/

HalfTime Technologies
Thread Factory is a component library for creating multi-threaded applications. Includes asynchronous calling convention that parallels COM+ with Begin_xxx and Finish_xxx methods and asynchronous error handling. HalfTime Active Show is a multi-threaded component library for adding graphics and sound to desktop applications. MouseTrap is a free component library that allows capture mouse events.
http://www.halfx.com/

Stubby
Stubby is a tool that enables programmers and packagers to add transparent support for optional run-time dynamic linking of shared libraries. This allows for the creation of applications that can gain additional functionality if a library is present, but that does not fail to load if the library is not installed. (Unless a function in that library is called.)
http://onegeek.org/~tom/software/stubby/

VCX Library
Audio streaming components and libraries for Borland Delphi and Visual Studio
http://lakeofsoft.com/

Libxslt - the XSLT C library for Gnome
Libxslt is the XSLT C library developed for the Gnome project. Libxslt is based on libxml2 the XML C library developed for the Gnome project. It also implements most of the EXSLT set of extensions functions and some of Saxon's evaluate and expressions extensions. People can either embed the library in their application or use xsltproc the command line processing tool. This library is free software and can be reused in commercial applications (see the intro). Includes a link to a tutorial.
http://xmlsoft.org/XSLT/

VCX Library
Audio streaming components and libraries for Borland Delphi and Visual Studio
http://lakeofsoft.com/

Windows Template Library (WTL)
A C++ library for developing Windows applications and UI components. It extends ATL (Active Template Library) and provides a set of classes for controls, dialogs, frame windows, GDI objects. [Open Source]
http://wtl.sourceforge.net/

Torch Library
A library in C++ for developing Open Source speech and machine learning applications. The code is under the Gnu Public License.
http://www.torch.ch/

libcw: C++ client/server application library
A C++ library for heavy duty networking applications. For Objects Management, Events, Signals, Timing, and Asynchrone Stream Devices.
http://libcw.sourceforge.net/