Anarchy Online - but you need to give it some time, it is very complex. It's an old game but it's one I am always going back to.

You can download the client and create a free account that allows you to play the original game (no expansions included), well, for free of course. I've posted some information in the Wiki if you're interested and there are many fansites with guides and of course the official forums: forums.anarchy-online.com.