What’s the next piece of open source software that Microsoft should embrace?

After the recent announcement by Scott Guthrie that JQuery would receive official support from Microsoft (http://weblogs.asp.net/scottgu/archive/2008/09/28/jquery-and-microsoft.aspx), many people see that as the heralding of a new dawn.

We missed out on NUnit (MsTest), NDoc (Sandcastle) and NHibernate (Linq to SQL/Entity Framework) for sure, but what do you think should be embraced next?

Post your answer here: