APC Back-UPS RS 1500 LCD
I got the APC Back-UPS RS 1500 LCD. PC Nation has it for around $190 with free 3-day FEDEX. The unit weighs 25 lbs., so free shipping saves you a bundle.
Some people on here have recommended the APC Smart-UPS SC 1500VA because it has better power conditioning, but it's also quite a bit more expensive --- cheapest I found was around $350-375.
The APC Back-UPS RS 1500 LCD connects to your MP via USB. No software installation is required for basic operation. The Energy Saver System Preference automatically recognizes the unit and allows you to configure how you want your Mac to behave when the UPS is running on battery. Very cool.