Egg cartons make excellent pots for seedlings. Fill with compost and plant seedlings in each compartment.
Remove the top and use the bottom as an office organiser. Place paperclips in one compartment, rubber bands, in another and so on.
Use as an artists paint pallet. Open up the carton, place a blob of paint in each compartment and place the brushes in the lid. Toss away after use so there is no messy cleaning up.