While there are many, many Open Source projects out there that I admire and use on a daily basis, these are the ones I managed to give back to (in no particular order). Please follow the links (clicking the project name will take you to the GitHub page) for more information.
TSPLib.Net is a C# .Net wrapper library for TSPLIB (a library of sample instances for the Travelling Salesman Problem and related problems from various sources and of various types collected by Heidelberg university) that provides complete access to all the information provided in TSPLIB95 as at 12 Jan 2014.
Ninja-IDE is a cross-platform integrated development environment (IDE) that allows developers to create applications for any purpose making the task of writing software easier and more enjoyable. It’s also a secret ninja agency but this doesn’t matter
QtWaitingSpinner is a highly configurable, custom Qt widget for showing “waiting” or “loading” spinner icons in Qt applications, e.g. the spinners below are all QtWaitingSpinner widgets differing only in their configuration
This library allows you to tap keyboard and mouse events to detect and record their activity even when an application is inactive and runs in background. It attaches to Windows global hooks, tracks keyboard events and mouse clicks and movement and raises common .NET events with KeyEventArgs and MouseEventArgs, so you can easily retrieve information you need.
This project is based on ASP.NET MVC 5 and can be used as an example of how to implement user avatar uploading and cropping. It uses jquery, jquery.form, and jcrop and it has some extra code to manipulate the image before saving it locally.