Plash is a system for sandboxing GNU/Linux programs. Plash's aim is to protect you from the programs you run by letting you run them with the minimum authority and privileges they need do do their job -- this is the Principle of Least Authority (POLA). Plash can run programs in a secure, restricted execution environment with access to a limited subset of your files.
Plash is geared towards granting authority dynamically. Sandboxes are lightweight and can be created for each instance of a program. A sandboxed program can be given additional rights at runtime via the FilePowerbox GUI.
Plash has been cited as an implementation of a powerbox on the GNU Hurd wiki and by the google-caja project