OGame is a free to play mmo strategy game set in space. Its UI consists only by images and text, but its gameplay is quite addictive. It's one of the oldest browser-based strategy games out there and it has a huge player base. You start with an undeveloped world and you have to expand it to a mighty space empire. To do so, you'll have to create an economic and military infrastructure to support your technology research and your defence (or offence)!