Developer's description:

Yahoo! Zimbra Desktop is the next-generation application that makes email easy again.

The free Zimbra Desktop client allows you to meld the online and offline worlds – storing and synching your email, calendar, contacts, files and documents in the cloud, yet having them locally accessible when on the road. Zimbra Desktop aggregates information across accounts (Zimbra, Yahoo! Mail, Gmail, Hotmail, etc.) and social networks (Facebook, Digg, Twitter, etc.) to help make communicating & sharing information easier than ever.

Key features:

* Email, contacts, calendar, and documents all in one application
* Syncs Zimbra, Yahoo! Mail and Gmail email, contacts and calendars
* Read email from any POP or IMAP email account including AOL, Hotmail or business email
* Works on Windows, Apple, or Linux desktop computers
* No limit to the size of your email storage
* It works online and offline
* Available in 20 languages

Feature details:

Email

* Easily compose, edit, delete, reply, or make drafts
* "Drag and Drop" messages into new folders or the Trash
* Supports plain text and html message formatting
* Add email signatures for each account and automatically reply with the correct "from" address
* Compose email even when you are not online-messages you send are saved in the "Outbox" and are sent when you connect again
* Supports any POP or IMAP email plus Zimbra, Yahoo! Mail, Gmail, AOL and Hotmail

Conversation Views, Tags, and Search

* Collapse email threads into a single Conversation View to simplify your inbox
* Tag messages as important from people you care about
* Quickly search your mailbox for pictures, documents or text-even with attachments
* Use Advanced Search to easily search by folder, date, person, subject or attachment
* Save searches as Virtual Folders- for example "All email with attachments from Joe"
* Now features built-in support for Yahoo! Inquisitor

Web mash-ups

* View addresses as Yahoo! Maps
* Automatically detect your location and find points of interest with Yahoo! Local
* Preview web pages at thumbnails instead of opening a browser
* See your calendar schedule from within an email message if you hover over a date
* Web Search powered by Yahoo! is built directly into Zimbra Desktop
* Automatically save pictures to Flickr

Contacts

* Store all your contacts in one place for all your accounts
* Create groups and tags to organize them
* Add photos to contacts
* Auto complete email addresses when composing email
* Easily import new contacts from other applications as .csv files or export contacts as .csv files for backup
* Syncs Zimbra, Yahoo! and Gmail contacts

Calendar

* Manage multiple color-coded calendars
* View calendars in Day, Week, Work Week, Month or List view
* "Fisheye" in the Month view let's you easily view and edit events
* Drag and drop' events to new days or to change times
* Import public web calendars (iCal format), such as sports schedules
* Invite others to meetings and view free/busy times
* Syncs Zimbra, Yahoo! and Gmail calendars