Yammer 

Yammer is an instant messenger developed for companies. By opening a community in Yammer, you can start to connect people working in the same company, and share content and files with them.

In order to start a network, you must provide an e-mail address whose domain name belongs to a company (not a Hotmail, Gmail or Yahoo! account). They will send you an e-mail to verify that the address provided belongs to you. Once done, you will be granted your own community webpage hosted in www.yammer.com/your company, and you can start entering the information you want to share. You can also invite other people to join your work network. They will need to have e-mail addresses with the same domain yours has.

You can download the Yammer Desktop Client in order to communicate with your co-workers easily. There is even a version that can be installed in mobile phones, to enable you to keep in touch with your co-workers when you are on the go.

This client works under the Adobe Air platform, so you will need to install Adobe Air in your system in order to make it work. The client will stay in your system tray as an icon, and will pop-up when you receive new messages.