Have extra egg cartons laying around at home?
Don't throw them out! Save them for starting your seeds! Poke a tiny hole in the bottom of each round, fill with your own compost or potting soil, and plant your seeds!
When your seedling is ready to be transplanted, just break apart the carton into pieces and put the whole thing into the ground.
Because the egg carton is biodegradable, it will break down once it's in the soil!