Agree with those - Wikispaces for me - but same sentiment in that it is easy to use for staff and students (and no age issues because teachers can create users).
Storybird is excellent (see the other thread for age comments!).
It's tricky picking favourites! Some of mine in this post.
That's far too many - you need to be really choosy!
Concentrate on a few that need you to explain more and give them links to easy ones they may/ will be able to explore themselves.
2 of the resources I build some pretty big projects around are GlogsterEDU and Fakebook. We create Presentations about Historical Figures from our 6th Grade Academic Standards and I use Fakebook as an extension. It is an outstanding learning experience for my students to approach Fakebook as if it were around when their Historical Figure was alive. All posts have to be historically relevant to what their Figure would actually say. Friends have to be who this person would have really been friends with.
I share resources several times a week on my blog if you would like to check it out!