Barbeque meatballs and cheesy taters.  In foil square containers, so they don't have to worry about returning anything. I always cover with foil and write the cooking instructions on the foil top with magic marker so that there's no questions when they take it out of the freezer or fridge to cook it. 

Also like to do a big casserole that uses burger, pasta, Rotel tomatoes, corn, cream of anything soup and shredded cheese with French's onions on top.

You always know that whatever the situation, they always appreciate whatever is brought in. It's the thought more than the actual food sometimes.