LongLivedMarshalByRefObject A MarshalByRefObject that lives forever

 Stopwatch This class is a System.Diagnostics.Stopwatch on operating systems that support it. On those that don't, it replicates the functionality at the resolution supported.

 BindingFlags Specifies flags that control binding and the way in which the search for members and types is conducted by reflection.

 AssemblyExtensions Extensions for Assembly that are not available in portable

 AttributeHelper Provides a platform-independent methods for getting attributes for use by AttributeConstraint and AttributeExistsConstraint.

 MemberInfoExtensions Extensions to the various MemberInfo derived classes

 Path Some path based methods that we need even in the Portable framework which does not have the System.IO.Path class

 TypeExtensions Provides NUnit specific extensions to aid in Reflection across multiple frameworks