We reviewed one of the beta releases of Thunderbird last December. The new Thunderbird includes several new features, including email tagging, message history, and a number of other interesting features.
This release also supports saved searches, so users can create searches for messages that match certain criteria -- like tags, date, priority, age in days, and so forth. Then just go to the search "folder" and see all of the messages that match the criteria.
Thunderbird 2.0 also has a feature to allow users with Gmail and .Mac accounts to access their mail by just providing the username and password, so no server or port configurations are necessary.
For all of the new features with 2.0 see the release notes on Mozilla.com. Official builds are available for Linux, Windows, and Mac OS X. The Moz folks recommend a 2.2 Linux kernel or better, a Pentium II or better CPU, and at least 64 MB of RAM. Unsupported builds for Solaris and other platforms are not available yet.