An offshoot of the Mozilla browser suite, Thunderbird is the open source organisation's attempt to create a stripped-back, cross-platform email client that's safe and secure.
Unlike the majority of cross-platform applications, it actually manages to feel at home on the Mac. The interface is clean, and while it's a whisker away from achieving Mail-like elegance, it looks superior to most other email clients, despite the occasional legacy/cross-platform quirk.
More important is whether an email client works well, and Thunderbird excels at most tasks.
The interface is simple to use and configurable (via plentiful but user-friendly options within its preferences); Thunderbird's search system is superior to Mail's, with powerful filters (usefully enabling you to base a filter on a selected message); and its support for different display types is first class.
Those interested in dabbling with HTML email will be interested to note that Thunderbird provides more than Rich Text support - its Compose window is effectively a basic web page editor.
Thunderbird does occasionally trip up. Its import options are lacking, only offering the choice of Netscape or Eudora, so moving over from another client can be tricky.
The lack of integration with Address Book also irks, although this is perhaps down to the application's crossplatform design. We're also rather surprised by signature creation relying on attaching external files; surely a built-in composer wouldn't be too much to ask for?
Overall though, Thunderbird proves in use to be a great, feature-rich email client, and the fact that it's cross-platform and open source means support is superb, with updates being regularly released.