I don't know if such a plugin exists, but it's possible. When you click thank/discourage on a post, the forum software knows which link you clicked. It lodges the karma but discards the info about the link/post. SMF is open source and could be modified to save that info. So someone could write a plugin that lists everyone who thanks or discourages a post, right next to the post. I suspect that kind of transparency would radically change the current ethos around karma, since it would quickly expose the shill hater accounts.