[kellros]

I'm a fan of extension methods as well.

The pitfall to avoid with extension methods is that you should keep external dependencies low (e.g. only depend on System and System.Core for common reusable libraries).

You might even need to split such reusable libraries based on security context (e.g. Medium trust, Full trust) and capability (e.g. IIS Classic, Integrated Mode ~ OWIN).

I setup a local NuGet repository to host my reusable libraries (for myself and team). The versioning allows you to make breaking changes as necessary without breaking dependent applications. It's also feasible to then write well documented and tested, reusable code. See: http://msdn.microsoft.com/en-us/magazine/hh781026.aspx