[LinuxUsers] virtual wikis

[Roger E. Rustad, Jr]

Does anyone know which wiki solution best supports "virtual wikis"? 
(Not sure if that's the right term, but that's what I'm calling it)

For example, say that you have 10 clients, and you want to make sure 
that each client can only see their particular company's stuff.  Is 
there an open source wiki solution that someone could recommend that 
easily segments them?