Michael Tymn points me to this interview with the founder of the "guerrilla skepticism" movement, which rewrites Wikipedia pages to remove "woo" and related abominations. I don't think these efforts amount to much; to the extent that Wiki becomes known for the one-sided presentation of controversies, it will simply become less of a resource for people who want to make up their own minds.
In fact, I think this is already happening. Increasingly, when people cite Wiki in an online argument, the riposte is that Wiki is too biased to be a credible source. This is true of controversies in general (politics, legal battles, scientific disputes, etc.), not just the paranormal.
An online encyclopedia edited by pretty much anybody does have some value, but it will never be authoritative, because the people behind it are not authorities.
One statement in the interview stood out for me:
I enjoy finding pages that look like the “psychic” wrote it themselves and then I come in with the delete button. It is really a powerful feeling.
Admittedly, it's a violation of Wiki policies for a person to write about himself, but notice that the Wiki editor merely thinks the page "looks like" it was written by the psychic. It may not have been; who knows?
Nevertheless, hitting that delete button is "really a powerful feeling."