A web service, still under beta, that quickly records your audio, stores it online (public), and allows you to embed it in webpages or link to it in emails.
If you are looking for ways to engage and communicate with online students, this matrix will quickly show you options. Many are available for students to set up themselves, thus making them good options for online student group work.
This is the help section of the Wiki software embedded in Blackboard. It has some clear explanation of how and why to use wikis (and blogs) in the classroom. Plus some case studies.