Guaranteed Unassigned IP Address?

For testing purposes, Is there such a thing as an IP address (or range of addresses) that is guaranteed to be not assigned?

A bit like the Americans have ‘555’ numbers for use in films, TV shows, etc…

As do the Brits:

addresses in 10.x.x.x and 192.168.x.x are reserved for local networks: they don’t exist on the global network, and whether they make sense or not on your local network is completely up to you. Or you APN provider in GPRS.