The hosting service is free, which means, small ads (google-ads) will show up on each pages bottom but no fee needs to be paid. After the short registration procedure you can easily create as much wikis as you need to have to.
It differs between private and public wikis, the latter appearing in the overview as well as in search results and the RecentChanges list.
If you don't wants to let wikihost.org do the hosting anymore, you can simple download your whole, fully running, wiki system as tar.gz.
The underlying WikiEngine is GeboGebo