The thrifty side of you wants to reuse plastic food containers, but the little voice inside your head is wondering "is this safe?" What about storage? Reheating?
My blanket rule is to never heat food in a plastic container. Heat can cause the chemicals in any type of plastic to migrate into your food. And there is just too much controversy about whether or not that's OK for you. Invest in some glassware with plastic lids and put your leftovers in those - remove the lid, cover the container with a paper towel or better yet, a plate, and nuke away! I love Frigoverre - I got some 8+ years ago and it's held up wonderfully. Check out places like the Container Store for ideas. Also, places like Home Goods, TJMaxx and Marshall's often have glass food storage containers for at good prices.
So how do you tell which plastic's which? Plastics are usually labeled on the bottom with the numbers 1 through 7. Those labeled 1, 2, 4, and 5 are considered safer; 3, 6, and 7 are considered less safe. The less safe ones can contain BPA or phthalates - nasties you don't want in your body.
Need more specifics? Check out the Mighty Nest's guide to plastics.