I continue to be amazed at how easy it is to build a fairly complex and robust development environment around VisualStudio.NET using open source tools. We are using NAnt to do our builds, Draco.NET for continous integration builds, and NUnit for unit testing.
I've just started experimenting with using a Wiki for internal product documentation, and so far it's working out very well. It's so easy to organically grow your documentation, with much less overhead than traditional document or content management tools.
Playing with Wiki is what finally led me here, to my own weblog. I know it's all the rage, and I've been avoiding it, but here I've finally caved, largely because I was impressed with the content on a friend's blog. If you are interested in .NET development, check out Scott Hanselman's blog at www.computerzen.com