DC Micro Development, Inc.
Cross-platform development libraries for C/C++ and Visual Basic. Products include libraries for data compression, TCP/IP protocols, dynamic html emission, and help authoring software.
http://www.dcmicro.com/

OptiVec: Libraries for Engineering and Science
OptiVec offers more than 3000 Assembler-written vector, matrix and complex math functions for PC language compilers, 2-3 times faster than compiled code.
http://www.optivec.com/

SGE
API that provides low level access to audio, keyboard, mouse, and display framebuffer across multiple platforms.
http://www.etek.chalmers.se/~e8cal1/sge/

CMATH
Assembler written complex number math library (single, double and extended precisions), intended as a high-quality replacement for the available C++ complex class libraries, and as an add on for Pascal or Delphi.
http://www.optivec.com/cmfuncs/

OpenML (Open Media Library)
A cross-platform standard programming environment for capturing, transporting, processing, displaying, and synchronizing digital media (2D and 3D graphics, audio and video processing, I/O, and networking).
http://www.khronos.org/openml/

WCSC
Serial communication libraries, drivers, and DLLs for all Windows and MS DOS that include Zmodem, Xmodem, and Ymodem file transfer protocols, as well as full modem control. Nine bit and Multidrop protocol for RS232, RS485, and RS422 serial communication supported.
http://www.wcscnet.com/

DMM_Windows_DLL
A Win32 library allows to adjust shapes of windows and to work with translucence and transparency of the windows. All the functions can be combined with one another so that interesting effects in the windows can appear. [Commercial]
http://www.dmm.cz/en/dmm_window_dll.htm

SuperCom TCP/IP
An optimized, thread based, Low Level Engine, which handles simultaneously connections, buffers data and generates events. Optimized threads reduce CPU load to minimum. A dual mode programming interface is provided (DLL and ActiveX). [Commercial]
http://www.adontec.com/supert_e.htm

Nine-9s
Protocol stack software for telecom and data communications protocols in source code. Also video and audio codecs.
http://www.nine-9s.com/

Cioina
Mathematical parser and symbolic differentiator (Windows .dll and Linux .so files), and Delphi, Kylix, C++ Builder, C#, VB using examples. [Commercial]
http://www.geocities.com/alexei_cioina/cioinaeval.html

DllDump
A collection of over 700 DLL files.
http://www.dlldump.com/

HtmLayout
A small, fast embeddable HTML renderer, HTML layout manager and HTML printing DLL for Windows. [Commercial]
http://terrainformatica.com/htmlayout/

DoneEx Software
Software tools for developing desktop applications, including: DoneEx AppBinder ActiveX - allows launch DOS or Win32 console programs from application and to capture the standard input and output data streams of launched program; DoneEx AppBinder DLL - help organize data exchange between program and any other programs which use the standard input/output streams; DoneEx INI-File Manager Library - allows manipulate with INI-file by the easiest way.
http://www.doneex.com/

Hunny Software
MIME tool kits for C++ and Java, .Net.
http://www.hunnysoft.com/

ConvertSpot
Provide COM/DLL components to convert PDF to XML, PDF to text, MS Word to text, PowerPoint to text, Excel to text, Excel to xml, Excel to CSV.
http://www.convertspot.com

ComISDN
Advanced TAPI Service Provider (TSP) for COmputer Telephony Integration and IP Telephony.
http://www.comisdn.com

SlavaSoft FastCRC Library
Fast, highly optimized implementation of popular checksum algorithms: CRC32, CRC16, CRC16C (CRC-CCITT). The library allows to calculate checksums for files, memory blocks, strings, streaming data. C/C++, VB examples. [Shareware]
http://www.slavasoft.com/fastcrc/

The Dynamic Link Library
Collection of 36000 Windows system files like DLL, OCX, VBX, INF and SYS.
http://www.dynamiclink.nl

DSX
Developer of ActiveX, Win32 DLL, and JavaBean tools for barcode solutions.
http://www.dsxtech.com/

RTF-to-HTML DLL
DLL/COM for convert RTF and text in to HTML, XHTML format.
http://www.sautinsoft.com/com/rtf-to-html_dll/rtf-to-html_dll.htm

Greenleaf Software
Serial communications libraries, components and tools.
http://www.greenleafsoft.com

EDSS/Models-3 I/O API
Provides the environmental model developer with an easy-to-learn, easy-to-use programming library for data storage and access, available from both Fortran and C.
http://www.baronams.com/products/ioapi/

DLL Files
Archive of DLL files for Windows.
http://www.dlldll.com/

Developer City: Free Programming Resources: Libraries
A variety of free libraries, with or without source code, for: accessing databases (with/without ODBC, SQL), encryption, graphics/images, compression, fonts, GUI's, 3D engines, 2D engines, and sockets programming.
http://www.thefreecountry.com/sourcecode/index.shtml

BinaryNotes
An ASN.1 framework for Java and .NET based on annotations/attributes. The framework has the flexible ASN.1 compiler, the runtime library that supports BER, DER and PER, and the Message Queues. [Open Source, LGPL v2/ GPL v2]
http://bnotes.sourceforge.net/

Guasi
The GUASI library implements a thread based generic asyncronous execution engine, to be used to give otherwise syncronous calls an asyncronous behaviour.
http://www.xmailserver.org/guasi-lib.html

MultiMedia SoftWare
32 bit DLLs for Windows: component for MS Windows Mixer controlling, component for any windows controlling. [Commercial]
http://www.mmsw.cz/