Online role-playing games in which a large number of people can play simultaneously are becoming increasingly popular and we will discuss some ways to put your Unity created MMO or MMORPG world online.
Developing a game that can be played by multiple people online takes planning and a solid story and you must also know what your game objectives (quests) will be as well as preparing player aspects such as game levels, experience points, player stats, attributes, gear and reward points.
There are many game engines that support online worlds but I recommend Unity which is a stable platform and is easy and free to use for indie developers. We write a lot about the Unity game engine here at Game Development Biz and that is because we love working in unity. Unity has a large support community, many professional third party assets to assist with game development and a core game engine that is powerful and easy to learn. Unity has many online features and some good MMO capabilities thanks to assets created by third party developers.
Making your game world available to online players will require a server. A server is a computer that is permanently connected to the Internet and configured to host games. You can use a computer you already own as a server or open a hosting account with a professional server hosting company. If you are using a hosting company it is recommended that you use a dedicated server to host your games but for a small number of players a virtual private server (VPS) may be enough to start with.
Most MMO server systems consist of a server component which will need to be running on the server and a unity asset that will allow players to log in and play the game as well as provide you and your team of developers access to tools for managing the server and other aspects of your online game from within Unity.
Connecting Unity to a multiplayer server requires some server set up and assets to be installed into your Unity project. We will cover some tools that allow you to create a multiple player server based game with Unity.
Atavism - Atavism connects players in your unity created world and also includes and impressive rage of plugin features that can be used for creating an MMORPG. The system is easy to set up and there are some good video tutorials demonstrating how to use the system with existing unity worlds. Atavism is a commercial product.
SmartFoxServer - A comprehensive platform for developing multiplayer games and applications. There is a free community version. SmartFoxServer is not specifically for Unity but has support for Unity. - http://smartfoxserver.com/
uMMO - uMMO is a Unity MMO Solution (uMMO) is a server and network management solution for Unity. This asset will help you to create a MMO from any single player game with a few clicks.
MMO Starter Kit - MMO Starter Kit is a good option if you want to have complete access to source code. Looks like this asset has potential but there is not a lot of documentation at this time.
The above systems are only a few handpicked options. There are other game kits for creating MMO games in the asset store you may want to investigate but these are the best options for creating a medium to large online multiplayer game.