.Net open source tools and components

I've been discussing about a specific .Net open source project in another thread and it occurred to me to start a new thread here. I'm hoping to get input about .Net open source projects that people have actually used in their own applications and production projects.

Here goes my own list.

Log4Net is by far the handiest logging component I've encountered. I've been using this since 2004 and I'm extremely happy with it.
Sandcastle Help File Builder proved a life-saver in assisting with help file generation.
I've deployed DotNetNuke in a couple of situations where I didn't want to just write another ASP.Net application. Easy to install, configure and extend.
iTextSharp has been a great component for creating PDF files. Difficult to work with initially, but with a lot of options.
I've used sharpSVN to integrate a solution with an SVN repository.
ZedGraph comes pretty handy when you need to create and show graphs.
#ziplib is a great component to help you work with reading and writing compressed files.
I've recently used FileHelpers to quickly implement code that reads/writes files in a proprietary format.
32.feet is indispensable when you're writing mobile applications that need to connect using blue tooth.
Smart Thread Pool provides a great implementation of a thread pool.