Unionfs is a "stackable unification file system, which can appear to merge the contents of several directories (branches), while keeping their physical content separate".
I worked on Unionfs for two semesters while at Stony Brook University. (well, I can't say I did much in the first semester!). I mainly worked on the Unionfs-ODF branch which when finished should give Unionfs a lot more power.
Unionfs home page