I disagree with Pxtl overall, but there are some high profile examples like Microsoft's Entity Framework coming from way behind and eventually overshadowing established open source ORMs like NHibernate.

On the other hand, creating a new ASP.NET MVC project with the official Microsoft template that ships with Visual Studio automatically pulls in open source libraries/frameworks like jQuery, Bootstrap, and Json.NET, not Microsoft-reinvented versions of those things.

I meant more specifically some of these "pain points" where undesirable behaviors stay in the library so changes won't break anything.

ASP MVC

...is open source.

I use ServiceStack under Mono as an alternative to Asp.net mvc and I love it.