I saw Majorie's extensive list of things she packs and I wanted to comment on things I find helpful.
Medicines are self-explanatory. Once I had a bad experience trying to find headache medicine in Paris. From then on no matter where I travel (even to my in-laws who live on a farm, yet closer to a grocery store than we do-go figure) I take a variety of medications. Not to be TMI but I ALWAYS carry GAS-X strips. LOVE these. If you haven't tried them, get them. They are soooo perfect for packing.
Next bring whatever headache medicine you like. Trust me if you are travelling to see in-laws you will need them. If you switch time zones you may need them. If you change altitude you may need them. If you go to a country that doesn't speak your language and you ahve to write everythign down so they can maybe translate because you botch their language so much no one understands you, you may need them. And take what you prefer. Because I GUARANTEE you, that if you like Tylenol 8 hour extra tremendous strength acetomenophine, the chances of a shortage in the area you are travelling in will be HIGH.
I take some sort of antihistamine. I am so not a doctor, so insert legal jargon about how I'm not telling you to do this, I am just saying what *I* do here. For many reasons, one of which is that it can put meto sleep when I am jet-lagged and can't fall asleep. But also because it is also a travel rule that I will be allergic to whatever freakish national/state flower is in bloom exactly during my trip!
Tummy meds. Is it's too fast or too slow. Enough said.
While you MUST take medications in your carryon they do not need to be in the stupid quart ziploc unless they are liquid. So try to avoid liquids.
Medicine, addendum First aid supplies. I take mainly bandaids, neosporin, and rubbing alcohol wipes.
Ziplock bags Especially if you are travelling near water or travelling to several places and need to pack wet things. I use them to put liquid/gel type souvenirs as well.
Wipes Don't to be Monk-ish, but however you think the cleaning staff cleans the room, it is worse. Immediately when you get in a room, wipe down the phone, the remote and anything that you will be touching frequently. I'm not saying totally reclean your room but I wear contacts so I wipe down the sinks JUST IN CASE I drop a contact in there.
Travel Alarm clock or Cell phone I have an Iphone so I use it for a variety of things, but I have really relied on it for wake up calls. I never trust the front desk.
Cell phone addendum For anything electric DON'T forget your chargers!!
Cell phone/Electronic addendum 2 International adapter/converter. And yeah by the way, if you don't have the converter...your adapter is worthless! Not that *I* have ever had this problem. (Insert No Scrubs song here)