There is a huge open source project that includes a gigantic library of over 1000 extension methods for C# and Vb.Net, this project has been previously hosted under CodePlex then it has moved to github, due to CodePlex shutting down. Read Brian Harry's article about Shutting down CodePlex!
The project's name is Z.ExtensionMethods under github, powered by Entity Framework Extensions and the open source community as well, you can start using this huge library of extension methods today, just add a reference from NuGet and enjoy 1000+ extension methods added by the community's wonderful people.
You can also download the source directly from github and check the implementation of these extension methods yourself, why not learn more about these extension methode, use them in your own projects or even add your own ones.