We need to have good, clear and continuous communication to be able to effectively collaborate with our team in any engagement. The selection of method and tools for enabling the smooth communication depends on the type of information, intended recipient, and project needs. In Agile teams, the importance of tools which enable to communicate is no less than any other teams.
Agile teams should be collocated, however if they are not Instant messengers (IM) enable Agile teams to communicate with each other instantaneously in groups and one-to-one communication. IMs are the most popular tools for Agile teams who wish to stay in touch and share stuff.
I use Yahoo and Google messengers. Yahoo is particularly beneficial to share files, etc because it uses LAN to transfer files which makes it faster and more effective. GTalk is better for light and speedy talks. You should choose the IM as per your needs and team’s comfort. Most messengers comes with features to save the conversation, this enables to track what was decided and when. Voice chat feature is useful if teams are split across the globe or they do not have a phone on their desk.
At the end, I would say that IMs are more effective for small/quick chats. Longer/complex conversation should be done in person or using video conferencing or over the phone (in that order).