Hummingbird rocks, in my experience. It involves a fundamental shift in the way people create and access documents, since it doesn't work with network shares. It also means that you have to enter the meta-data associated with the files every time. However, it does have very strong permissions, access controls, and versioning support, and would likely solve your problem, since you can prevent those who don't need access to a document or project from access, or even viewing that the document exists. On the down side, it's fairly expensive. (In our organization, implementation was at least 5-figures, and probably 6) and it requires a lot of support and baby-sitting (1/2 to 1 FTE, with an organization of about 500).