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Zimbra rounds off this list of open-source, free desktop email clients. It works on Windows, Mac or Linux machines.
Unlike most of the others on this list, Zimbra can integrate communications from social-networking services such as Twitter (pictured), Facebook and Digg.
Like the rest of the bunch, it supports multiple accounts. Naturally, it comes with email services such as syncing, forwarding and auto-responders.
However, it is perhaps one of the more fully featured clients: it has advanced options such as viewing email searches as a virtual inbox (for example, all email with attachments from 'Ben'). It also plugs into Yahoo for 'web mash-ups', which show addresses on Yahoo Maps and allow you to preview web pages as thumbnails before opening them, among other features.
Screenshot: Ben Woods/ZDNet