Even though our helpdesk ticketing software is not open-source, we do open-source some of it's components as reusable libraries. Here are some of them:
SharpGoogleVoice - an open-source library for Google Voice for C#. We used it to send some of the SMS messages to the helpdesk agents when a new ticket arrives (currently available in US/Canada only).
Visual Studio unused images finder - not exactly a component, but a Visual-Studio plugin that helps you find unused images in an ASP.NET web-application project. Simple right-click the project and select "find unused images" in the context menu.
MailSystem.NET - it's not completely Jitbit's but it's a project we contribute a lot to. It's an IMAP/POP library for the .NET framework.
WYSIWYG BbCode editor - a "what you see is what you get" editor that outputs BBCODE instead of just HTML
Resx-Translator-Bot - translates resx-files automatically using Google-Translate, nice for multi-language .NET-apps.
Log2SQL - a handy little tool to convert webserver logs into a SQL database you can run queries against.
Cmd + T for Visual Studio - The Command-T extension provides an extremely fast, intuitive mechanism for opening files with a minimal number of keystrokes in VS. It's a version of "Cmd + T" feature from Textmate and Sublime Text (Mac OS X text editors).