Here's a list of them to help you turn your single player game into a MMORPG or another type of MMO.
Eclipse- www.freemmorpgmaker.com - Very popular, no coding, but sometimes difficult to understand with thousands of INI files, helpful forums.
BYOND- www.byond.com - Need to learn simple programming language called DM, pretty easy to use, bad forums but a great guide to help you.
Game Maker- www.yoyogames.com - Need to purchase full edition to make multplayer games, don't need to know code, very easy to use, but a lot of limitations.
Realm Crafter- I think you have to pay for it, but pretty cool.
there's some more, have to dig up the links...