Orion's Belt -
www.orionsbelt.eu
After playing dozens of space browser games, i'm current currently playing Orion's Belt.
It's a Space Browser games with an unique battle system (where the strongest is not the one who wins). The universe is also excellent. You have fog of war and line of sight like in a RTS. It has also a quest system and, according with what i eared, their are implementing Artificial intelligence. So you will have mobs wondering the universe and you can battle with them.
The battles also gave origin to tournaments were you can battle with other players just for fun.