A massively multiplayer online game (also called MMOG or simply MMO) is a video game which is capable of supporting hundreds or thousands of players simultaneously. By necessity, they are played on the Internet, and feature at least one persistent world. They are, however, not necessarily games played on personal computers. Most of the newer game consoles, including the PSP, PlayStation 3, Xbox 360, Nintendo DS and Wii can access the Internet and may therefore run MMO games. Additionally, mobile devices and smartphones based on such operating systems as Windows Mobile and Google's Android, as well as the Apple iPhone are seeing an increase in the amount of MMO games available. MMOGs can enable players to cooperate and compete with each other on a large scale, and sometimes to interact meaningfully with people around the world. They include a variety of gameplay types, representing many video game genres.
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Many MMO's have a free limited version. Runescape allows you to play for as long as you want in a small world which takes up about the equivalent of about a square mile. There are about 16 missions. All of which can be completed with the skills available. It makes for an easy start to get into the game. It helps you to get leveled up before getting involved with the big time paying players. It can take weeks do causal play to plateau. Note: This section was written based on early experiences from about 3 to 4 years ago. Runescape seems to have improved greatly since then I will be giving it a second try soon. I have joined Runescape as a free player once again. I was surprised that the game has change very little but not disappointingly so. The games graphics are still poor but to make them better would have required a massive overhaul that would have shut the site down for weeks and probably put a few nerds in the hospital with withdraw symptoms. The game does however have an HD option which provides slightly better texture to the terrain. Even thought the game hasn't changed much I am looking forward to playing further and eventually goin back to being a member instead of a F2P (Free-to play) player.

Also some dedicated fans have started a Wiki which is helpful to new users and returning players like me. It is slow but it hasn't given me any problems on my laptop.

Disney created a MMO called Pirates of the Caribbean Online. The free gameplay available doesn't allow for the first mission you are given to be completed. Free players share the same worlds as paying players leading to many popups tellingt you you are about to enter a member only area and that you can't go any farther. It takes 3 days of casual play to reach a point where you can't do anything.

Runescape.com

Second Life