Indev Software has updated MailTags (http://www.indev.ca/MailTags.html), a tool that adds the ability to tag email messages for organization work flows and sophisticated searches to the Mac OS X Mail app, to version 2.5. The new version is compatible with Mail 4.4.
MailTags is shareware with a 21-day trial period. Registration is US$29.95, though version 2.5 is a free update for registered users of version 2.x. Educational and volume discounts are available.
MailTags is Universal Binary so runs natively on both PowerPC and Intel Macs. It requires Mac OS X 10.4.7 or higher.