Nothing if you have a friend who has some webspace that's capable of running php, as I have.
We have a German Wiki that we have used for our Exalted campaign, and it works very well - so far, we have 257 artikles dealing with the campaign, including writeups of PCs, information on NPCs and locations, campaign diaries, and more. I can highly recommend it - it has been an exceedingly useful for our game.
My friend has also created an English-language Wiki which is indended as a repository for information for all groups who wish to playtest Urbis - so far, one group is interested, and I hope more will join up in the future.
Hosted by PBWiki.
It's free.
"As easy as making a peanut butter sandwich", hence the name of the host...
Bonus: You can use HTML tags for more variety.
My players don't contribute much to the content, but they often use it as a reference.
I installed my own on the SWRPGNetwork server for my Age of Worms campaign. Very easy to install and set up, though to be frank I didn't put a ton of work into it. My players were primarily in charge of updating it, though I believe they stopped about the 10th adventure, even though we completed the campaign before I moved.
I built my own Mediawiki and hosted it on my work machine, but it's nothing spectacular, as it's still getting content added to it. I like it quite a bit, very easy to maintain.