The Yahoo Zimbra Beta allows all 263 million Yahoo Mail customers to access mail and calendars offline.
Zimbra Desktop Beta Version 3, available July 24, will let users run their Yahoo mail as desktop POP or IMAP mail client software. Previously this required a Yahoo Pro account for $19.99 per year.
The Zimbra open -ource software also allows users to read their AOL Mail and Gmail from within the same desktop software. The calendar in Zimbra Desktop supports the iCal standard used by Apple and Outlook.
Microsoft has already delivered a desktop client for Hotmail-Windows Live Mail-and Google has been talking about making Gmail available offline with its Gears technology, although it hasn't shipped the offline client yet. The Zimbra client is available for Windows, Mac, and Linux, while Microsoft's Live Mail is Windows-only.