The idea of a community driven source of useful extension methods would itself be useful. I'm assuming that's the point of this thread? How would this work, practically?
Actually the modest idea was to just collect it in here, but that's of course very disorganized.
Your assumption is actually an interesting idea. Imagine ambient intellisense for extension methods out there - perhaps in a repository on codeplex.
So you dot (plus something) on a Control and find yourself with a list of free ambient extension methods. If you use the extension method, Visual Studio will transparently import the implementation code. The list will be prettified by WPF to allow documentation
expansion as well as implementation expansion.
Of course on something like String or Object, the number of community methods could become extreme. You'd probably also want a rating and commenting system for these.
I hope BCL is gradually refactored to separate out methods as extension methods when they really don't need to be part of the implementation. This can also provide null-safety.
And actually the BCL team can actually use ambient intellisense to be inspired here (1 cent for every use of this possible marketing term for me please heh)
And of course ambient intellisense in Visual Studio will help prevent duplication of work because people will first try ambient intellisense and then implement a solution (and possibly directly inside Visual Studio submit to the community).
One might also think about extension methods with dependencies on other extension methods. Often one good thing does not come alone and so you want a means to express that if you say A you also need to say B.