Components

Alvas.Net
Offers components including Alvas.Suite, Alvas.Audio, Alvas.FileControls, Alvas.ShapeForms, Alvas.Controls, Alvas.Scriptor, and Alvas.Labels.
http://www.alvas.net/

ASPPainter
.NET component can be used to create new and edit existing images. Complete source code is available for developers. Free and commercial versions.
http://dotnet.asppainter.com/

CenterSpace NMath
.Net components suite contains interfaces to BLAS and LAPACK, random number generators, anova, regression, probability distributions, sparse matrices, hypothesis tests and descriptive statistics.
http://www.centerspace.net

Coalesys, Inc.
Focusing on .ASP.NET User Interface controls.
http://www.coalesys.com

Dart Communications
Offers a web-based HTML editor which enables users to create rich-text content in a browser as well as TCP and SNMP components.
http://www.dart.com/

decloak
.Net components including: dual listbox with re-ordering, dependent listboxes, click click e-mail reports.
http://www.decloak.com/Products/NET/

DotLucene
A port of the Jakarta Lucene search engine. [Open source, Apache Software License]
http://www.dotlucene.net/

EaseSoft Barcode
.Net Windows Forms control and ASP.NET control for creating barcode images.
http://www.easesoft.net

etive.com
Provides .NET components to use in both ASP.NET websites, and winform applications. Including dotEtiveFTP, dotEtiveSocket, dotEtiveTreeListView, etiveAspPack, etiveCreditCard, etiveDNS, etiveMessenger, etiveWHOIS.
http://www.etive.com/

Extreme Optimization
Numerical components for .Net includes complex numbers, vectors and matrices (BLAS and LAPACK), numerical integration and differentiation, solving equations, curve fitting, special functions, statistics.
http://www.extremeoptimization.com/

fyiReporting Software: RDL Project
A fully functional report and charting system based on Microsoft's Report Definition Language (RDL). [Open source, Apache]
http://www.fyireporting.com/

Industrial DOT NET, Inc
Local IO assembly provides generic access to instrumentation data.
http://industrialdotnet.com/

InRule .NET Rule Engine
A rule engine that provides a framework for non-technical logic maintenance and deployment within server, web-based, and desktop applications.
http://www.inrule.com

KineticaRT
A collection of .Net components, including Windows .NET Forms сontrols, logic components, server components.
http://www.kineticart.co.uk/Default.asp?page=Products/KineticaRT

Lanap BotDetect
.NET control to protect website from spambot (form robots) with asp.net captcha code image. ASP.NET security solution to prevent automated registrations.
http://www.lanapsoft.com/

Naxos Development
Offers a collection of .NET user interface controls and components. [Commercial]
http://www.naxosdev.com/

NLinAlg
A .NET class library for working with algebra structures such as matrices and vectors. [Open source, GPL]
http://nlinalg.sourceforge.net/

Olvio IT PlusSuite
Include .Net controls, editor controls, layout manager, notification controls.
http://www.olvio.com/WebModules/Products/PlusSuite/PlusSuite.aspx

Parole Lite
Generate pronounceable passwords. By 4ASPNET.
http://www.4-asp.net/products/parolelite.aspx

Perpetuum Software LLC
Specializes in the development of .NET reporting, charting, OLAP, gauge and instrumentation components for Visual Studio developers.
http://www.perpetuumsoft.com/

PortSight Secure Access for .NET
Manages users and their access to WinForms and WebForms applications, Web Services and Web content. It features user roles, permissions, auditing, delegation and import from Active Directory.
http://www.portsight.com/Products.aspx?AliasPath=Products/Secure%20Access/Secure%20Access&CultureAlias=en-US

PortSight Task Scheduler for .NET
Immediate or planned execution from ASP.NET. It allows processing of long-running ASP.NET pages, running command line utilities or other server-side tasks without page time outs and under a privileged account.
http://www.portsight.com/Products.aspx?AliasPath=Products/Task%20Scheduler/Task%20Scheduler&CultureAlias=en-US

Quantum Whale
Editor.NET component is an advanced code editor allowing integration of a highly flexible edit control in your .NET applications; Outbar.NET is a component pack allowing incorporate a side bar control into any .NET applications.
http://www.qwhale.net

Registry Toolkit .NET
Extends the functionality of the Microsoft registry classes. By ComponentSpace.
http://www.componentspace.com/registrytoolkit.aspx

SharpSpell
AJAX-enabled ASP.NET and Desktop spell checker component for .NET developers. [Commercial, trial version]
http://www.tachyon-labs.com/sharpspell.aspx

SSDN: Simple Service Discovery for .NET
An open source service discovery mechanism for .NET remoting. Also provides a simple naming and directory service.
http://www.michaylov.com/spiro/ssdn/ssdn_overview.html

Subsembly
.NET components based on e-Banking standards (FinTS, HBCI), e-Government standards (OSCI) and smart cards (PC/SC, CT-API).
http://subsembly.com/

Trace Listeners .NET
A cyclic file trace listener. By ComponentSpace.
http://www.componentspace.com/tracelisteners.aspx

USPExpress Math Parser .NET
Parse and evaluate scientific, engineering and mathematical expressions on-the-fly. BY Unisoft Plus, Ltd.
http://www.unisoftplus.com/uspexpress/

WebCab Components
.Net components for financial and mathematical applications.
http://www.webcabcomponents.com/dotNET/

Xtras.Net
An online reseller of third party .NET components.
http://www.xtras.net/

ZXValidator
.NET component for credit card numbers validation. It currently supported credit card types: Visa, MasterCard, Amex, JCB, Diner's Club, Discover/Novus.
http://zamov.online.fr/zxvalidator/