BTsys
BTsys is a lightweight desktop bug tracking system, written using C# and ADO.NET for accesing to database. Easy to install and learn how to use. Distributed under the terms of the GNU General Public License.

BugBye
BugBye is a web-based bugtracking system developed using ASP.NET technology and C# as scripting language. It offers all the features needed to manage improvement, bugs, and so on. It also provide statistics help-desk management, and further options which allow the user to simplify all the stage of project's development and maintainance.

Buglog V2
A bug logging and tracking solution with file attachments and history of comments.

BugNet
An open source web based bug / issue tracking application programmed in C# and asp.net. Main goals are to keep the codebase simple, well documented, easy to deploy and scalable. Major features to include: Generic Data Access Layer CSS layout Mono Support Web Services.

BugTracker.NET
BugTracker.NET is an easy-to-use web-based bug tracker written using ASP.NET and C#. It's fairly stable: Hundreds of people (maybe even thousands) are using it, and they have done a good job flusing out the bugs in it. It is distributed under the terms of the GNU General Public License.