Is there any good bug tracker based on .NET which is free?
Two bug trackers I know:
BugTracker.NET is a free, open-source, web-based bug tracker or customer support issue tracker written using ASP.NET, C#, and Microsoft SQL Server (or its free cousin, SQL Server Express).
BugNET is an issue tracking and project issue management solution built using the ASP.NET web application framework. Email notifications, reporting and per project configuration of fields and values allows efficient management of bugs, feature requests, and other issues for projects of any scale.
ASP.Net issue tracker is a simple alternative.
You can download it from http://www.asp.net/downloads/archived/starter-kits/issue-tracker/
Some commercial ones will let you use them for free if you're just one developer. Axosoft will give you a free single user license to their OnTime software. (Although I found it to be overkill for me personally.) SourceGear's Fortress issue tracking software is also free for single users.
Countersoft offers a free license for up to 5 users for their product Gemini. I think they also have a free license for open source projects.