One of my favorites...

I love using Titanpad with classes!  Titanpad allows students to work on the same document at the same time.  It doesn't even require users to have login, which as a teacher I love.  I created an account for myself using my classroom as my source.  Having an account allows me to create as many Titanpads as I want!  Plus I can control the security of them by making them public, but requiring a password that I set.  Then, I share the link and password with my students to allow them to work together on one document.
The only downfall is that there is a chat feature on the side.  My students LOVE using the chat feature to write about ridiculous things.  My creating titanpads for smaller groups, rather than the whole class, it eliminates some of this.  Also, by setting guidelines about what can be chatted about helps as well - give them a purpose to focus their chats on.
Another feature I love is the export feature to a word document.  It makes it super easy to use and save!  They are super fast to set up and in just a few minutes you can be up and running with the document.

Wikis in Action...

So we are off and running with wikis so far!
The students seem to like it and we've had only one override issue, but with the student accounts we can go back and look to see if the student actually posted and then see how it got overridden.  Fantastic!
One student commented that they thought it was like facebook, but for education.  I'm not sure if I completely agree with that, but they have been eager to try and use the wiki.  I think I will continue the use of the wiki for the rest of the year and see how it goes!  I definitely like the option of not having a stack of papers to lug back and for with me and that I can see everything from my laptop!