Batteries and water seem to be the first things people wished they had. Food and ammo were close behind. Baby diapers and medicine were also quite frequently mentioned.
When Hurricane Hugo came a hundred miles inland and trashed Shaw AFB, I snagged the battery out of my car, wrapped a little speaker wire around the battery terminals, and attached the ends to the back-up light fixture I stole from the car's trunk, and had electric light in my living room. My neighbors asked how I got my lights on, I showed them, and lights started popping up all over the housing area... Three days later, we got our power back, and I put everything back the way it was.
heavier wire would have let me use more lamps, but what we had was good enough for a D&D game after a day's disaster relief work.
I now keep a spare battery in the house, with wire and such, and I have 3 different APC UPS systems; 1100VA, 700VA, and 300VA. I've also got a 5W solar panel from Harbor Freight to recharge batteries if needed. (I've got GEL-CELL batteries in both cars, now, too.) My computer now is a laptop, and I have quite a bit of info on CD-rom, and textfiles on the computer for later reference. I can build a lathe from pipe parts, for example, if I need one. I've got a walking-beam bandsaw from an article in Fine Woodworking. It can be hand-cranked, or motorized.
We also keep quite a bit of canned food, and 2-liter soda bottles of water, as well.
It gets pretty cold here, in winter, so we have a kerosine heater, 20 gallons of K-1 in 5-gal. containers, etc. Haven't needed that yet, but it could happen, so we are prepared for it.
I like the Mossberg 500, but if I could afford it, I'd go for a Remington 870... maybe after I finish the college thing...
For medications you need regularly, build up a surplus. I've got terrible allergies, and asthma, so when conditions are good, and I don't need my daily meds, I don't take them. I'm working on building up a year's supply, but aren't anywhere near there, yet. I can get by on benadryl, which is OTC and cheap, if you buy the store brand, so we keep quite a bit of that around. Useful for bee & wasp stings, too. Get the elixer, and you can apply it topically, too.
If you want some pointers on things that might be useful, and you like to read Science Fiction, read Niven and Pournelle's "Footfall" and "Lucifer's Hammer" for some hints.
You might also check out some of the reenactor groups. Some of them practice old-tech that can come in handy in a long-term or short-term disaster.
Batteries I can understand, but water? They had plenty of time to fill up every container available, and those in houses had 30-50 gallons of water in their water heaters.