Expect much more on this in the next few weeks but Sony has made a slightly gimped version of Killzone 3 multiplayer free for all, well all in the USA.
Next week a download will appear on PSN that will enable you to download the MP version of the firey-shooty-killy-zoney game for no money - and that will include all maps.
It won't contain all the available skills or weapons, and it will also introduce a level cap. You will, however, be able to get access to all of these Multi-Player bits and bobs for an additional $14.99, yes, in the USA only.
More details at the PSBlog