Zimbra, the Yahoo open-source messaging and collaboration unit that tends to get overlooked amid all of the Microsoft-Yahoo merger talk, is beta testing offline access for the Yahoo Mail, Gmail and AOL Mail Web e-mail applications, as well as some Zimbra tools.

Leveraging POP (Post Office Protocol), IMAP and the iCal standard for the calendar app, Yahoo Zimbra Desktop Beta 3 is designed to let users access their e-mail applications of choice without an Internet connection.
Such functionality is increasingly becoming table stakes for productivity apps because it helps business travelers work on their laptops while in flight, or simply in places where there is no Internet connection available.

Google in particular has been active on this front, adding offline access for Google Docs, with similar plans for Gmail and Google Calendar this year. Microsoft's Windows Live Mail app works offline.

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