It's almost inevitable that you're going to forget something when you're packing for a trip, but save for personal identification, most can be replaced with little irritation or expense.
CheapFlights.com put together a list of some of the most common forgotten items and what to do after you realize they're not in your Samsonite. Phone chargers are always a worry, but unless you're traveling to Antarctica, you can pick up a replacement at most electronics stores or use a temporary one, courtesy of your hotel. Umbrellas are definitely an easy item to forget, especially if you live in the Southwest where precipitation isn't a common concern. Bringing a small one everywhere you travel comes in handy — you can leave one in your bag even when you're home — since buying one on the street or even a drug store can be pricey.
Other items on their list that resonated with us were prescription medications — if you're out of the country, you're kind of screwed (but CheapFlights has some ideas) — and corkscrews and bottle openers. How often have you bought a bottle of wine or a six-pack only to come back to your room and bang your head against the hotel/motel/hostel wall?
Read the full list here.