I have always packed along a supply of Ziploc-type bags, especially the quart size for 3-1-1 use, but also snack, sandwich, and gallon bags as we rent apartments and cook and need to preserve leftover foods.
At home I have been using Bee's Wrap products for quite some time so this year on our European trip I took along two Bee's Wrap Sandwich Wraps. They came in handy many times for wrapping a bit of leftover cheese, an entire loaf of bread, or even covering a bowl of leftover soup. Since they are reusable I am not using or carrying along as many plastic bags.
Wash-Eze is a product that came up in another thread and I also highly recommend them. I always have a few in my luggage and sometimes cut them into small pieces for handwashing. A little costly, but better than having to buy a bottle of laundry soap in a foreign country and leave 3/4 of it behind when you leave.