Jim Newkirk and I have been doing xUnit.net for 7 years now (and for Jim, NUnit for many years before that). You could say that open source is part of our blood, and when we left Microsoft, we made sure that open source would continue to be part of our daily efforts at Tier 3.
Fast forward 15 months: Tier 3 has been acquired (and is now the CenturyLink Cloud Development Center), and our first major open source effort Iron Foundry has been accepted into the Cloud Foundry Incubator project. Lots of great developers are working to ensure that you can write .NET code against a Platform-as-a-Service stack that doesn't lock you into a specific vendor.
Today we are proud to announce our second major open source effort: ElasticLINQ.