The APC 675VA Battery Backup UPS is back on sale at $55 for one day.
This and other 600+ VA UPSs normally sell for at least $65. A similar APC with a slightly smaller battery sells for $65.
UPS keeps electronics online for a limited time during a power outage. 675VA can provide battery power to things like: A cable modem, router, or wifi access point for about an hour or more. A fish tank filter for about 45 minutes. A desktop computer and monitor for about 25-30 minutes. Beware that your $55 investment will last for just 3-5 years. All batteries have an expiration date, and this battery is rated to last only until its 3-year warranty ends. A replacement battery costs $40, and is easy to install.
APC UPSs alarm loudly during a power outage, and these annoying beeps can be permanently silenced by plugging a computer into the UPS's USB port and configuring with their PowerChute software (Windows only, no Mac).