XiaoXiao 9: Fight Man
This classic fighter has been around for a couple of years, and yet it is still one of the best Flash fighters I've seen. Stick figures are the characters in the XiaoXiao series of games, and there are plenty of them to challenge you in XiaoXiao 9: Fight Man. Default controls are the arrow keys for movement, the [Z] key for boxing, [X] for jump, and [C] for protect. An options screen before the game begins allows you to configure different keys to suit your taste. If fighters are your thing, then XiaoXiao.
Created by Zhu Zhiqiang. Thanks go to Matt for reminding me about this great stick figure fighting game.