One of the frustrating things about my local recycling center is that they only recycle #1 and #2 plastics, and that means that all my containers for sour cream and cottage cheese have to go into the trash. I just can't do it-- so I save them to use for "tupperware" or freezer containers or whatever comes up that I need a simple container for. Whenever there is a use for something that is normally thrown away, I'm all over it. But even so, I still end up with more than I need (I guess I really love my sour cream!) So with all the painting I've been doing, I find they make great paint containers for the "cutting in" brush. If I need to stop before it's empty, I can snap the lid on and save it for later, and if I'm done I can toss it without having to wash it.
Pros: They are free. (I don't have to purchase a paint cup.) It puts plastic at least to multiple uses before being thrown away.
Cons: Doesn't have a handle like the store purchased cups. It still is plastic going into the trash.