Fallensword

Fallensword is perhaps the best free browser-based mmorpg I have ever come across. It is quite popular, with a player base of nearly 2 million players and is based in a fantasy world with dragons, goblins, etc. The pictures for various monsters and equipment are drawn superbly and look as though they are the work of professional artists. The game is very simple to learn with a large focus on PvP, but it should be noted that you will have to plan your skill allocation carefully (I suggest using the Fallen Sword Guide http://www.fallenswordguide.com/). You have a limited amount of stamina which recharges every hour (you can increase this by using "points" which can be either bought with standard in-game currency or using real money). Actions such as attacking or moving around in the in-game world use up stamina. Because of this stamina-based system, this game is great for casual gamers who do not have much time on their hands. However, this does not mean that gamers with a lot of time on their hands will not enjoy this game. While the game is free to play, points are required for many actions (such as crafting or forging items) and are used more frequently as in-game currency at higher levels than gold is. Points are usually purchased with real money, but you can also buy points with gold or by completing offers provided by the site (however, many of these require a credit card and are US only).

Pros:
Very competitive, large focus on PvP
Large playerbase with a friendly community
Artwork is fantastic

Cons:
Players who buy in-game currency with real money have a large advantage over those who don't
Very easy to "ruin" your character if you take points in the wrong skills (guides are recommended)

Play Fallensword now! http://www.fallensword.com/