Which is the best open source .NET tool you have used? Please list one tool per answer.
Mono (Open source .net implementation for OS X, Linux and mostly anywhere else someone is interested in running it).
CruiseControl, for build automation
EDIT: Nant, mentioned previously in this thread, is used to automate builds. CruiseControl automates Nant (fires it off when there's a checkin, can notify you via a try icon when someone has broken the build, etc.). You can leverage your investment in Nant scripts with CruiseControl.
GhostDoc is handy for XML meta-data commenting in Visual Studio.
Regionerate:
http://www.rauchy.net/regionerate/
A great addin for VisualStudio that organizes your code into regions, helps keep your stuff clean.
Selenium is a test automation tool that provides a few different ways to automate testing of web applications. (via a GUI with Selenium IDE, or via NUnit with Selenium Remote Control).
TreeSurgeon!
http://www.codeplex.com/treesurgeon
Sets you up with a ready to roll Nant build file and project structure ready for CruiseControl
WatiN - http://watin.sourceforge.net/
Awesome test automation tool similar to Selenium.
Has a recorder too:
http://watintestrecord.sourceforge.net/
Good for IE and Firefox testing since version 2.0.
Not sure if this applies but: MySql Net Connector (for using MySQL as a db for .NET Programs)
NUBuild - The simplest of all build .Net tools out there.