Ertha is an open source GIS whose main focus is to guarantee easy extensibility and straightforward integration of custom modules. It provides basic geographic rendering and topologic analysis and exposes the main data structures loaded from shapefiles.
The Anywhere Location Search allows for location searches using a wide range of inputs (address, city/state, zip code, search string, IP address, landmark name, etc).
ArcDeveloper.net is a user community centered around the ESRI ArcObjects platform. This project will focus on a set of community utility classes for use with ArcObjects.
An Editor for the OpenStreetMap-Project
.NET Framework GPS communications class library for communicating with GPS devices using NMEA 0183 and/or Garmin protocols. Written in C# with some Win32 interop (not quite mono compatible).