Space Abyss

Space Abyss is a free, sandbox MMO.

Some standout features are:
Explore a galaxy, landing on planets, exploring and salvaging crashed ships.
Battle monsters and collect resources on planets. Even take over planets!
Build more advanced and larger ships, and build bases on them.
A classless system - swap bodies to meet the needs of what you are doing (Walk around a water planet as an octopus!)