Mozilla Thunderbird is a mail client for reading your POP3 or IMAP mail. With a simple add-on installed, Thunderbird will also retrieve webmail from the most popular webmail services like Hotmail, Yahoo, etc. Thunderbird supports multiple accounts, mail tagging, spam blocking and mail and address book import and export with other popular mail clients.
Thanks to the add-on structure, similar to the one in Mozilla Firefox, you can extend the functionality of Thunderbird without unnecessary bloat. Examples of useful add-ons are Lightning which adds an integrated calendar (not unlike Outlook); Google Calendar provider that allows synchronization between Google Calendars and Lightning; WebMail that allows checking your webmail in Thunderbird; etc.
pro: free, open, intuitive interface, extensible, good compatibility.
con: slow startup, no tag-based browsing, no default calendar integration.
verdict: [rate 3.5]