Jack @ ASP.NET

I like free one, so I only list top 5 of my favorite ones.

1. Twitter Update: (by Krishnan)

Allow you to add up to 3 twitter accounts and tweet according to the blog you post.
In the options page enter default Twitter account user name and password, which will be used when there is no matching twitter account for a blog is found.
Add blog home page Urls in the text box (seperated by | if you have more than one) and enter the twitter account details of the corresponding twitter account

2. Blog This for Firefox:Adds a button to Firefox which starts a new Windows Live Writer blog post prepopulated with content and title from the current web page. Blog the whole page, or just selected snippets. Interacts with other registered plugins to parse and structure web content where appropriate. Blog quickly when you find something of interest on the web.
3. Syntax Higlighter for WLW: Add smart content editor syntax higlighter 2.0 support for Windows Live Writer.
4. Facebook Live Writer Plugin: Prompts you to share a link on Facebook whenever you publish a new post.
5. Tag Generator: A tool to automatically generate tags based on the content of your post. Supports Technorati, WordPress and Blogger

• Rabbit MQ – Rabbit MQ is a scalable message/queue AMQP server with WCF/C# client (free)
• Twillio – This is an awesome commercial programmable VOIP infrastructure. Free accounts available (commercial)
• ejabberd – This is one really scalable jabber/XMPP server (free)
• fyiReporting – This is a free reporting tools for .Net based on Report Definition Langauge (RDL) (free)
• Dotnet OpenID – C# library for OpenID authentication (free)
• Tweetsharp – An excellent library for Tweeter API (free)
• XML-RPC.Net – This is *the* library to make XML-RPC calls (free)
• Jabber-net – This is the only client library for Jabber/XMPP protocol
• Mono RelaxNG Validator – This is the most viable RelaxNG validator reader for .Net (free)
• Quartz. Net – Enterprise job scheduler for .Net (free)
• Topshelf – Windows service application framework (free)
• Facebook Developer Toolkit – If you want to develop a Facebook App on .Net, use this (free)
• Gitsharp – Library for Git version control (free)
• SharpSVN – A library for Subversion Client API (free)
• Math.NET – A mathematical open source (MIT/X11, LGPL & GPL) library written in C#/.Net, aiming to provide a self contained clean framework for symbolic algebraic and numerical / scientific computations. (free)
• Linq to Twitter – Linq to Twitter
• Sharp SSH – SSH Implementation in C#

Another great project in codeplex: http://facebooktoolkit.codeplex.com/

This toolkit is provided as a Facebook Client Library similar to Facebook’s PHP Client Library or Facebook’s JavaScript library. The goal is to enable .NET developers to quickly and easily leverage the various features of the Facebook Platform. This toolkit has evolved over time with input from the community and from Microsoft. The latest release (v3.0) includes new architectural improvements and provides an asynchronous interface for using the toolkit from Silverlight and from WPF.

The main entry point is the API (Facebook.Rest.Api) class in the Facebook.dll assembly. This class wraps the Facebook REST API and provides an easy to use interface for calling the different methods currently available in the Facebook API. We’ve also provided samples and tools for helping develop Facebook applications in the various .NET platforms including: ASP.NET, Silverlight, WPF and WinForms. Additionally, we’ve provided all the source code for the API, components, controls, and samples for you to explore.

The toolkit is comprised of the following core assemblies:

• Facebook.dll: This is the main assembly that will be used by all applications. This has all the logic to handle communication with the Facebook application. This assembly also has specific support of XAML applications (Silverlight and WPF) to enhance the Facebook platform to make databinding and data caching easier.
• Facebook.Silverlight.dll: This is the Silverlight version of the main assembly that will be used by all Silverlight applications. This has all the logic to handle communication with the Facebook application. This assembly also has specific support of XAML applications to enhance the Facebook platform to make databinding and data caching easier. The REST API in this assembly is Asynchronous only.
• Facebook.Web.dll: This assembly should be used by Canvas applications. The main functionality supported in this assembly is to encapsulate the handshake between the Facebook application and a canvas application (both FBML and IFrame)
• Facebook.Web.Mvc.dll: Provide a support building canvas applications using ASP.NET MVC. Separated from Facebook.Web.dll to avoid all developers from needing to install the MVC bits.
• Facebook.Winforms.dll: This assembly provides support for writing Facebook applications using Winform technology. This provides a Component that wraps the API to make it easier to use from Winforms. This also contains some user controls to help display Facebook data easily.