Online fighting games are not quite as popular as RPGs or strategy games but there are still plenty of good ones for action fans.

Little Fighter 2
Graphics: 2D
Download: Yes
Pay or Free: Free
Arcade-style fighting game with 2D graphics. This game must be downloaded before playing.

Street Fighter Online
Graphics: 2D
Download: Yes
Pay or Free: Free
Arcade-style fighting game that can be played in your browser or downloaded.  

Robot Rage
Graphics: Yes
Download: No, Browser
Pay or Free: Free
Build your own robots and then fight against other robots worldwide. Stats are tracked and the best players are ranked.

Get Amped
Graphics: 3D
Download: Yes
Pay or Free: Free
3D action fighting game that combines the best of old school arcade fighting action game and sports challenges such as soccer and basketball set against various backdrops.

Toribash
Graphics: Yes
Download: Yes
Pay or Free: Free
Toribash is a turn-based fighting game. Create your own martial arts movies in single player sandbox mode, or join the competition in the multi player modes. Focus is on tactics rather than reaction.

Bots 
Graphics: Yes
Download: Yes
Free or Pay: Free but has item mall.
Multiplayer online fighting game by Acclaim. Players get to upgrade their bots and compete in several different modes. 

Rumble Fighter
Graphics: 2D
Download: Yes
Pay or Free: Free
Fighting game that allows players to battle it out with a wide variety of items, combo attacks, and counter attacks. The game has transformation items to spice up the combat.

Grand Chase
Graphics: 2D
Download: Yes
Pay or Free: Free
A Free-to-Play online adventure game with adorable, anime-inspired avatars and a rich, medieval setting. The game serves up several fun modes, including an exciting Player versus Player (PvP) function modeled after old school arcade titles.