One of the biggest emergency situations that I encounter is power outages (granted I live in hurricane country, but it should be of some concern for everyone). Its good to have a back-up plan in case of an extended period without power. Depending on your budget a couple of battery operated air pumps, a decent power inverter to run from your car battery, or an electrical generator are options that all work well. My power inverter has saved my livestock on several occasions, and it only cost $50
If you are paranoid about tank leaks/failures a couple of large rubbermaid containers are good to have on hand (can pull dbl duty as quarantines too), as are LOTS of towels, perhaps even a wet/dry vac.
Backup equipment in case of failure is nice to have as well, although this can be fairly expensive. I keep a few old bulbs around.. just in case one breaks ect, I also have a spare ballast in case of failure, and a mag-drive I can drop in as a back up quickly if for some reason my external pump dies on me.