Sometimes I come across some handy sites, and my favorites are free.
Authorea let's multiple people collaborate on writing articles. It's free but you have to sign up.
Miraheze.org is a free Wiki site using MediaWiki, the same wiki language that Wikipedia uses, which is also the most popular wiki language. A wiki lets you write markup, it's easier than HTML, and it supports formatting, links, images, videos, etc. To make your own free wiki, go here.