Well given the amount of space based games being presented I thought I would share one I played a LONG time ago (2001-02ish).
Mankind
This is a true MMORTS based in space. You start with a lone basic builder and a couple hundred settlers. You then build up your bases, cities and ships and can wage war or peacefully trade ships depending on what you wish to do. Its been a long time since I played so I cant vouch for how densely populated it is but there is always room somewhere for a new player to start working. Also by its nature you can only do a limited number of things per day. That sounds restrictive but its not like you have 50 moves what do you want to do. Its more like the more advanced structures and research take significant amounts of time to build from a couple of minutes for the starting structures to 10080 Minutes(thats a full week) for the most advanced buildings. However you can have multiple workers all working on separate buildings. Honestly during the time I played having setup a large base its not an overly restrictive problem because you can just make new workers and set them to work immediately. Note that if you are patient enough you don't even need to set up a base, just do missions for the Imperials (neutral party that you can buy and sell stuff to) or one of the other various ways to make money. Then with enough credits you can place an order with a trader for pretty much anything else in the game. If you do decide to play I recommend starting a demo account and researching everything you can then head out to where you want to set up your base before paying for the account.(Unfortunately you have to pay)