Aliasource has released a plugin to import PST files in Thunderbird. It is released under the GPL, and is part of Aliasource's OBM collaboration tool, which also has a community site.
OBM offers a broad range of possibilities, from Groupware to CRM, from messaging to project management, and even a Synchronization framework.

Aliasource offers services around OBM, but all sources can be downloaded. OBM is built on solid, proven software such as OpenLDAP, Cyrus-IMAP, Postfix, Clam/AV, Spamassassin, Funambol, apache, Mysql or Postgresql, PHP, etc

As seen on Linuxfr