Age of Ascent
by Illyriad Games.
A massively multiplayer space shooter that you can play right from your browser with no download required (it uses Microsoft’s Azure servers). The highest number of concurrent players so far is almost 1000, with a much higher potential ceiling.
Currently, every other Saturday you can play the PvP dogfighting aspect, check the website for the countdown timer to see when it opens.
It’s a bit difficult to record nice GIFs for games with realistic-ish graphics (that play nice with Tumblr), so take a peek at the trailer if you like.