UnionFS alike on windows, Virtually merge folders

"Windows 7 has introduced the concept of Library, which is functionally somewhat the same as UnionFS. From How to Use Windows 7's New Library Feature:

You could say Libraries are virtual
folders. You add folders to Libraries,
where you can view and access the
contents of each added folder from the
single Library window. A Library
itself can’t contain any files as they
aren’t real folders, but they offer
one-stop access to a collection of
folders.

Here are several ways you can make use
of Libraries:


Add folders that are scattered throughout your hard drive(s) to
Libraries.
Create custom Libraries. For example, when I start a new writing
assignment, I can create a new
Library. I could add related folders
such as the one that contains the
publication’s guidelines, folders on
previous work that I think I’ll want
to reference, and folders containing
any downloaded research.
Add network locations (shared folders from other computers) to
Libraries.


For some more detailed info, see:

Windows 7 Libraries .NET Sample Library – an Overview
Understanding Windows 7 Libraries"