It's the egg sac of a praying mantis.
They look like styrofoam and you can find them on trees, weeds, twigs, brush, etc.
We have a few different kinds of mantids out here, but I think this sac is from the praying mantis - Mantis religiosa.
In the spring, they'll hatch out by the hundreds [I hope] and eat plenty of other bugs.
Someday I want to see the hatching....maybe next year?