Sega has opened the Phantasy Star Online: Blue Burst beta and is allowing people to download the game client for free.
PSO originally came out for the Dreamcast back in 2000. It offered Diablo-styled dungeon crawling and hack and slash fighting - all with an outer space/science fiction feel, and all while online with up to 4 players at once.
you can download the client here: https://www.phantasystaronline.net/