Sort of like a two dimensional Second Life, but without the thriving sex industry, 8BitMMO is a top-down, java-based sandbox construction game currently under development at Archive Entertainment.
Its world, built entirely from scratch by users, supports up to 250 concurrent players in a persistent environment, who can either craft on their lonesome or join with others to create collaborative constructions. A "grief protection system" prevents malicious ne'er-do-wells from jackin' with your craft, should you choose to build alone. There's also "basic" PvP and PvE combat, as well as "amusing quests and unusual enemies" like the Lawyercat.
Despite still being under construction, the latest build is available for free at the game's official site.