I already have an account with several of these tools. My favorite is Skype and I have been using this both personally and professionally. It's a great way to keep in touch (and cheap). I've also used it to meet with other educators in different aspects. We've worked on conference planning and professional development. It's a great way to meet with people from all over the world.
Another tool I've started using is Diigo. I think this would be fun for students to use when working on projects and research. It will also be very helpful for me to use with graduate work.