Yoomba (Israeli Start Up) is a free service that lets anyone with an e-mail address send and receive instant messages and VoIP calls without creating any new user accounts.
Needing what? only a functioning e-mail address, users can quickly download the Yoomba client from the company website, and without any registration can contact other e-mail addresses over chat or VoIP call by clicking one of two buttons that automatically appear next to e-mails in your e-mail client.

Yoomba
In order to chat or make calls, however, both users need to have Yoomba: if the other address contacted does not have Yoomba, he/she will get an e-mail directed to Yoomba’s download site. Yoomba is compatible with many e-mail clients inculding GMail, Yahoo Mail, Hotmail, and Outlook.
Why this service is wonderful? The service eliminates the need for phone numbers. Users only need their email account and an email address to make a call or IM… HEY! what are you waiting for? go download The Yoomba client !!.
Yoomba was originally founded in Tel Aviv, Israel as one of the world’s first open communications companies. What makes Yoomba magnificent? Yoomba is unique to offer an open standard available to any community, in other words, it connects people universally across e-mail address as opposed to over closed user networks which often involve registration and restrictions.
And check this out!!! Yoomba reached over 500,000 users in less than a month(!!!) from its July 2007 launch.
Yoomba Site: http://www.yoomba.com/
Nocamels also on