There's some people that use Back-Up UPS's on their setups. They're nice, but depending on what model you have, they only have a limited battery life (also depending on what all you have plugged into it).
I don't use one personally (not on my tanks anyways) because of the fact that if I have a long power outtage, the battery would loose it's charge in few hours, and I'd still be out of power.
They're good for short power lapses, but not long-term. I prefer battery-powered air pumps for long term power outtages. With those, I can replace the batteries as needed and don't have to worry about how long the power outtage lasts.