DotNetOpenAuth.OpenId.RelyingParty by: Andrew Arnott

Increase conversion rates to your web site by lowering the bar to create an account with your site, by allowing customers to use accounts they already have with other sites, such as Google and Yahoo!

DotNetZip by: Dino Chiesa

DotNetZip is an easy-to-use, FAST, FREE class library and toolset for manipulating zip files or folders.

DSharpPlus.CommandsNext by: Naamloos, afroraydude, DrCreo, Death, Axiom, Emzi0767, and contributors

Advanced command framework for DSharpPlus.

Elasticsearch.Net by: Elasticsearch Inc. and contributors

Exposes all the api endpoints but leaves you in control of building the request and response bodies. Comes with built in cluster failover/connection pooling support.

elmah by: Atif Aziz

ELMAH with initial configuration for getting started quickly. ELMAH (Error Logging Modules and Handlers) is an application-wide error logging facility that is completely pluggable. It can be dynamically added to a running ASP.NET web application, or even all ASP.NET web applications on a machine, without any need for re-compilation or re-deployment.

elmah.corelibrary by: Atif Aziz

Core library for ELMAH (Error Logging Modules and Handlers) without any configuration.

EmailValidation by: Jeffrey Stedfast

A simple (but correct) .NET class for validating email addresses. Supports mail addresses as defined in rfc5322 as well as the new Internationalized Mail Address standards (rfc653x).

EntityFramework by: Microsoft

Entity Framework 6 (EF6) is a tried and tested object-relational mapper for .NET with many years of feature development and stabilization.

EntityFramework.Migrations by: Microsoft

Legacy package, Migrations is now included in the EntityFramework package.

Enums.NET by: Tyler Brinkley

Enums.NET is a high-performance type-safe .NET enum utility library

EnyimMemcached by: Attila Kiskó

Memcached client library

ExcelNumberFormat by: ExcelNumberFormat developers

.NET library to parse ECMA-376 number format strings and format values like Excel and other spreadsheet softwares.

ExCSS by: Tyler Brinks

ExCSS is a CSS 3 parser for .NET. ExCSS makes it easy to read and parse stylesheets into a friendly object model with full LINQ support.

ExtendedNumerics.BigDecimal by: https://github.com/AdamWhiteHat

Arbitrary precision decimal class

FastEndpoints.Swagger by: FastEndpoints

Swagger support for FastEndpoints.

FastMember by: Marc Gravell

In .NET reflection is slow... well, kinda slow. If you need access to the members of an arbitrary type, with the type and member-names known only at runtime - then it is frankly hard (especially for DLR types). This library makes such access easy and fast.

FFMpegCore by: Vlad Jerca, Malte Rosenbjerg

A great way to use FFMpeg encoding when writing video applications, client-side and server-side. It has wrapper methods that allow conversion to all web formats: MP4, OGV, TS and methods of capturing screens from the videos.

Fizzler by: Atif Aziz, Colin Ramsay

Fizzler is a W3C Selectors parser and generic selector framework for document hierarchies.

FluentScheduler by: Jim Geurts

A task scheduler that uses fluent interface to configure schedules. Useful for running cron jobs/automated tasks from your application.

FluentValidation by: Jeremy Skinner

A validation library for .NET that uses a fluent interface to construct strongly-typed validation rules.