Zojirushi Mr. Bento Stainless Steel Lunch Jar
I know several people who swear by their Mr. Bento Lunch Jars. It's a lunch box designed for the classic Japanese lunch, with stacking containers inside a vacuum-insulated stainless steel jar. It's a really perfect system, foolproof and well-sealed. It stays hot (or cold), and the containers inside are microwaveable.