What can you use styorfoam egg cartons for?

You can use them as plant starters!

Here are some more cool ideas from fellow Hometalkers http://www.hometalk.com/search/posts?filter=Egg%20Cartons

as a plant starter, you really should use the biodegradable paper egg cartons. Once the seed takes off, you can cut into small sections and without disturbing the roots, put the carton section along with the soil and plant right into the ground. if you are gardening with a hydroponic method, then the styro will work well, because you will be rinsing off all the dirt before entering the plant into the hydroponic system.

I use them as a paint try to put my acrylic paints in when painting.