The C# IDE lets you easily clean up your .cs files’ using sections by right-clicking in the editor and selecting “Remove and Sort Usings”. Unfortunately, there is no such command for removing unnecessary assembly references from a project.
Judging from the reviews pages of each product, all of them seem to share the problem that sometimes they remove assemblies even though they are required for compilation.
(This problem may occur if code in assembly 1 calls assembly 2 which returns an object defined in assembly 3, but assembly 3 is not referenced by assembly 1.)
Reference Assistant has a preview of the remove operation (screenshot), but I am not sure about the other products.
What is your preferred tool for this operation?