Hrm, so I'm curious if Glitch counts as a sandbox mmo? It's F2P and browser based, plays a lot like Harvest Moon or Animal Crossing in mmo format. There's death but it's frivolous and not damaging in the slightest. You otherwise have control over creating new regions, buying and designing homes with furniture you built and designed, populating the world with trees, animals, etc.. of your choosing. It's definitely not a competitive MMO though and once an area has been established, besides killing the natural resources in that area off or replacing them with other things, you cannot mold it in any new way, which is why I'm unsure if it qualifies as a sandbox.
Trailer:
http://www.youtube.com/watch?v=b6O5QXj6n18
Housing:
http://www.youtube.com/watch?v=OSOfOeJpSb8&context=C4391199ADvjVQa1PpcFNBQD4wmUsEV6_nwmXLXgS4Sul0jqdyyHk=