Progress Telerik JustMock
Mock everything like non-virtual methods, sealed classes, static methods and classes, as well as non-public members and types everywhere even members of MsCorLib.
Mocking .NET System members (a.k.a. MsCorLib) can be a daunting task. But with JustMock you can arrange against any MsCorLib member or function including DateTime.Now without any additional set-up.