Simplest would be to attach a 120 Volt NC (Normally Closed = ON) relay
to the compressor's two electrical feed wires. When the compressor is
running, the relay is activated and its contacts are open (=Off).
When the compressor has stopped, the relay closes and lets the sump
pump, e.g., run.

What you are seeking is akin to this
http://www.kk.org/cooltools/archives/001087.php
but with a reverse action. You could purchase a strip like this and
plug the frig into its power sensing socket. A NC relay could then be
plugged into the controlled sockets. Would do the same as above but
save you the trouble of messing with the refrigerator wiring.

Ron Hardin wrote:
Is there a power-strip like device for use with portable
generators that turns off outlets when other outlets are using
a certain number of watts?

Like you'd want to turn off the refrigerator while the sump pump
cycles, to keep the total watts under say 2000; but mostly the
refrigerator stays on.