similar to Ethafoam, an inexpensive but very light foam commonly used for packing. It makes for a hard ground pad, but should work well for hammock insulation.
The more expensive blue pads (WalMart and REI) are softer and more conformable. I believe they have a higher R-value than ethafoam as well.
Open cell foam is sponge-like: It will absorbwater readily, and takes some time to fry. Closed cell may have surface cells that appear to be "open", but in reality are just a portion of a gas-blown bubble that was cut during manufacture.