Ning

http://www.ning.com/

Create your own "facebook" type of community.  Free, but requires registration and can be restricted to certain users.  Easy setup.

Wet Paint

http://wikisineducation.wetpaint.com/

Another very popular wiki host.

Slideshare

http://www.slideshare.net/

Do you have a great powerpoint presentation that you'd like to share?  Or do you want an interesting powerpoint for a project?  An easy way to pass on powerpoints, publicly or privately.

Authorstream

http://www.authorstream.com/

You can use and share powerpoints, video or audio.  Includes features to upload directly to YouTube, send greeting cards, or discuss presentations live.

Moodle

http://moodle.org/

Online blackboard-type site.

Skype

http://www.skype.com/

Free internet calling audio or video.

Voxopop

http://www.voxopop.com/

This is a synchronous voice discussion group, but very targeted to language learners.  You can search by topic category and choose between open, private and restricted groups.

Prezi 

http://prezi.com/

Very wild alternative to giving traditional presentations. Could be used for planning speeches and essays.

Yodio 

http://yodio.com/

Upload photos and narrate with your voice from a microphone or phone!

Audacity

http://audacity.sourceforge.net/

This is THE best recording software available for FREE!  Great for recording and editing voice tracks, music, etc.

Pics4Learning

http://www.pics4learning.com/

I'm always looking for free pictures when making powerpoints or exercises, and this is a nice resource, arranged by theme.