Here is an interesting product that we have used to allow different users to access a computer system whose screen is visible but cannot otherwise be tampered with. Its called Transparent Screen Lock and it now has support for Smart Cards - this includes the CAC cards used by the military.

You can download a copy from http://www.e-motional.com/TScreenLockSC.htm

Definately not something everyone would need but definately a one of a kind software application