Mail and Messaging
Zimbra
Zimbra is the leading open source based collaboration product. With desktop, AJAX and HTML clients accessing a central standards based server Zimbra offers a robust enterprise class collaboration environment at a fraction of the cost of the industry standard alternative. Community (free) and Enterprise (additional features and support) editions are available.
http://www.zimbra.com
Gmail
Use of the gmail API makes integration of this cloud product with a standards based business infrastructure possible. This functionality is especially useful for large enterprises which can avail themselves of free/very low cost options.
http://google.com/a