A simulator that is a sandbox and also an island. Island sandboxes are good for testing objects that could be "lost" (such as a bouncing ball) or somehow accidentally "spread" to other simulators (such as scripted wildlife).
The current island sandboxes are Sandbox Island and Sandbox Island Extension. They were formally located to the west of the other Sandbox Sims until Linden Lab relocated them. Currently the six sandbox sims have been placed together. They are (clockwise from the upper right) Sandbox Cordova, Sandbox Goguen, Sandbox Newcomb, Sandbox Wanderton, Sandbox Island Extension, and Sandbox Island.