I use a half liter (i think) kleen kanteen metal insulated mug to keep things either hot or cold like soup or pasta salad. i dont like carrying salad to work as it wilts and we have a pretty cheap salad bar in our downstairs cafeteria.

Otherwise, i tend to use things that are screw top and an insulated lunch bag in my backpack. i dont like carrying a backpack much but this one has one of those mesh springbacks that minimizes sweat.