Mesh bags you get citrus fruit in - we wrap these round each other into a big bundle and make them into pan scourers (they are awesome at removing scrambled egg!)
Plastic tubs (e.g. the ones you get hummous in) we use for everything - storing leftovers in the fridge, picnic containers, paint pots.
And today we have upcycled some egg boxes and tissue paper into "jellyfish" - MJ is so chuffed with them - they're "swimming and landing on the seabed" as I type

So maybe not that ingenious, but I wanted to pick your brains... so what do you reuse?