Master of Orion II is definitely one of the best of its kind (and I still have it installed
). However, the best 4x space strategy game I ever came across was Space Empires V; the only complaints I have against it are its poor graphics (by today's standards), and weak AI (though it has multiplayer options).
I have recently started looking for another game of this type, and came across Cosmic Supremacy, which is an online based version of these games - it doesn't seem bad: you can "design" your own ships; there are a good number of technologies to research; fair amount of buildings available - but it seems quite limited, when compared to the games mentioned above. Still, if someone wants to give it a try:
http://cosmicsupremacy.com (it's not browser based however, and does require a small download to work).
Anyway, if someone finds a very good, online, massively multiplayer, feature rich, (preferably) browser based, game of this kind, let me know.