Yoomba is a new web service that lets you call or text chat with anyone in the world who has an email address (that means practically everyone) - no registrations or complicated setup required.
When Yoomba spots an email address on your screen, it will add two icons - one for making voice phone calls and the other one for instant messaging. (see a screenshot from my GMail inbox below) - this works for Outlook, GMail, Yahoo and other web pages.
Here's how Yoomba works - you provide your email to Yoomba and download a small client which is pre-activated and won't ask you to setup an account or login. When you click the Yoomba button near any email address, an invitation request is sent to the other person - he or she can download Yoomba and connect with you almost instantly over voice or text messaging.
Yoomba is based on P2P technology very similar to Skype. The one very advantage here is the ease of use and it bypasses the need of creating a separate id for chatting / web calling - the email address is id of you and your online friends.
Currently there's option to import contacts from other IMs like Yahoo, Skype or Gtalk but you can add your Microsoft Outlook / Outlook Express contact in a click. You can also see the online status of your contacts inside Outlook. A promising service for sure.