Sandboxie is, like the name indicates, a sandbox for browsers. This is a Windows only application. This is like a quarantined space for your risky browser to run.

You can't access some websites if you have an updated, safely patched computer. Specifically, the federal government demands visitors use old versions of web browsers. Doing this risks virus and trojan infection. So, using a sandboxed virus collector (like IE6) is the most safe way to do this. Actually running an antique browser without added protection is just a bad idea.

Google this issue and you'll see Xenocode and Spoon mentioned. If you are running Windows, the default online and virtualized Spoon browsers allow you to launch a virtual browser environment to run various browsers, including older and less safe browsers.

Spoon also offers Studio. It allows one to run virtualized browsers (and other apps) on your PC. It is commercial and licensing is thousands of US dollars.