I installed a "basement watchdog" brand after a power-falure scare (water
was right up to the top of the sump when the power came back on). It claims
that it can run continuously for 24 hours. Since my pump would never need
to run continuously (it can clear the sump in about 30 seconds) I should get
several days out of it.

I've never needed it for an emergency yet, but it was very cheap insurance
(about $150, installed myself). Since it's a completely separate pump, it
will also work if my main pump has mechanical failure.

The water-based backup systems weren't a good option as my sump isn't
anywhere near a water line. I also didn't like the idea of wasting all that
good water, plus I've heard horror stories where people actually flooded
their basements with city water when a check valve failed...

-Tim