Do you have a class library for all extension methods?
Do you copy & paste extension methods from one project to another?
Do you have different class libraries for different extended objects (like stringExtensions.dll, IComparebleExtensions.dll etc')?

Personally, I find it best to have a different dll for each assembly - so for instance I'll have a StringsExtensions class and an IComparebleExtensions class in one dll called <companyName>.System.Extensions.dll.

What do you prefer? Why do you prefer that?