Last I checked MOSS required SQL Server Standard Edition or better.

A SQL Server Sandbox is a usually a database on a SQL Server Test or Development box where developers and/or DBAs can play without affecting a development or test database. You will even find that DBAs and developers will sometimes set these up on their own desktop or laptop systems using SQL Server Express (free) or SQL Server Developer Edition (approx. $75 or less depending on where you buy it).

The benefit of putting on your desktop or laptop is that you don't have to worry about others messing in your sandbox.