Wikispaces is my preferred wiki platform. I got onto it via a tip from local TAFE IT trainers. But wikis aren't self evident tools for everyone who approaches their coal face. So I've found a couple of videos that will walk you through the main tricks you need to know.
Standardly now for most of my blog projects I'm teaming them up with wikis -- usually Wikispaces wikis -- as a means to extend the utility of each blog. Like blogging its a learning exercise to discover how to use them. But, I assure you, wikis are an easy DIY.
Editing a page
Note: if the wiki is public you don't have to sign in to edit the wiki.
Creating a new page