Most of the tools in the main part of this Wiki so far relate to Social and Group Teaching. Here we are also looking at tools which can also be used for group learning, which may be a co-operative activity which is not teacher led.

Pending Tools
VoiceThreadThis is a (commercial) tool which allows a multimedia discussion to develop between a group of users around a topic (thread) There's a good (if long) page about applying it educationaly here Digitaly Speaking pageand an educause page about it here: http://net.educause.edu/ir/library/pdf/ELI7050.pdf
Thanks to Lisa Dawley of the teacher stream ning for this.
There is a very good Wiki devoted to teaching with Voice thread here: http://voicethread4education.wikispaces.com/home

Google+ is an obvious new addition. its particular advatage is the segmentation of users, so that a class could be in a circle with you, without seeing the content you post for your family or friends.
Cops
Ning This is a (mostly commercial now) tool for creating online social spaces. These spaces typically have discussion boards, a personal home page for each users, common resources that one may download etc.Ning used to be the great free tool for constructing online communities of practice, however they switched over to a fee paying model recently and this has led to the demise or migration of a number of active groups of the site to Ning alternatives:
Grou.ps 1st 100Mb of usage is free, and Spruz another free alternative.
A personal note here. There are a lot of dead networks out there. This doesn't seem to be just a matter of achieving a critical mass of users. Unless your group is going to answer a real need for online comunication and sharing it seems you may just end up with another dead cop on your hands. (cop= Community of practice)

Social Bookmark Tools
Social bookmark tools allow you to build collections of bookmarks which you may share with others. The classic here is Del.icio.us, now Delicious, however tools like Zotero which also include a citation engine for incorporating your bookmarks and citations into word processed documents are also significant contenders. This is just touching the surface:
To do
Evernote,For web mobile and document indexing and commentary.
Jing,Capture a computer image, post a link and start a conversation.
Diigo,....Bookmark and highlight web pages, send links to who ever.

Educationally you could curate a set of bookmarks for your students, and then share this collection with them.
Screen capture tools, there is a list of eight here:
http://networkedblogs.com/zBLIN

http://en.wikipedia.org/wiki/List_of_collaborative_software For the more technicaly adept, this is a list of wikipedia list of collaborative and joint project software.