NEStalgia is a retro-tastic 8-bit multiplayer online RPG
Dubbed by its own developers as "Dragon Warrior 3 meets World of Warcraft," NEStalgia lets you wage turn-based battles using a multiplayer system while also having a full-featured quest system, plenty of loot to find, and even an Auction House for item trading.
After you join the game, you create a hero and then play against friends and strangers. Customization is a big part of the game, as you can dress up your character in whatever clothes and armour you like, choose whichever class suits your playing type, and even create your own guild -- which comes with its own customizable cape.
NEStalgia is free to download and play, however you need to register with the BYOND game system so you can save your games and stats. If you want to support the developers, you can go for an optional yearly subscription that will give you a few extra, but non-essential features, such as additional character classes and even more ways to customize your character's appearance.
To play NEStalgia, head to this page and start your download. You'll need Windows, a 500 MHz processor, 512 MB of RAM, and (obviously) an internet connection.