BackUPS 650. I lost power once in the middle of a backup, and I was able to finish the bu and shut down normally. Without it, I might have lost both the internal and BU drives. And a BackUPS CS 500 for my peripherals. I could get by running everything off the 650, but splitting them up insures that I'll have enough juice for everything to finish whatever I'm doing.
They work so well I even have a BackUPS CS 500 for my TV and DVD player. I think they're invaluable. Everyone should have one.