MailTags is one of the best plugin I ever used for my Mail application. I found this plugin when I had to organize my inbox with follow ups and todos. The plugin intergration with the application is very good.
Integrated Interface – Use MailTags’ convenient side panel to add tags keywords, project, priority, notes to your messages. Use contextual menus to tag several messages at the same time. All of MailTags interfaces feel built in, not added on.
IMAP Support – Save your tags to your IMAP servers so you can access them from multiple computers.
List Display – Display your tags in extra columns in the message list view. See due dates as they approach.
Works with Mail’s Smart Mailboxes – Build smart mailboxes that search your messages for keywords, projects and even due dates.
Works with Mail’s Rules – Set tags automatically as you receive Mail by using MailTags actions when creating a Mail rule. Use with Mail Act-On to create keystroke rules to tag messages.
Works with Mail’s Search – Search for your tags by using Mail’s search field, narrow the search to keywords, projects or notes. Use simple and/or logic to find tags.
Tag Mail 3.0 Notes – Create a note for yourself with Mail 3.0 notes feature and tag it with projects, keywords, tickle dates and priorities for future reference. Find those notes in smart mailboxes.
RSS support – Tag news items for future reference. Ideal for workflow management where a RSS subscription notifies you of forum posts, support requests or tasks.
iCal Support – Create To Dos and Events in iCal directly from Mail and maintain the link between the original message and the items in iCal.
Send Tags in Messages – Not only can you tag your outgoing mail for your purpose, you can send tags with the messages to create an informal workflow with your recipients.
Incoming Reply Tagging – Automatically copy tags from earlier messages when receiving replies. Maintain tags in a discussion even if the other correspondent doesn’t have MailTags.
Add functionality with Extras – MailTags new extensible architecture means that you will be able to add functionality to MailTags by adding an Extra. Planned extras include components that work with Google Calendar, TaDa list and other web based task/event management system. We plan to publish an Extras SDK so others can extend MailTags.
Applescript Compatible – Construct Applescript work flows that access your MailTags data to use them in other applications.
All for just 30$. Real worth for money 🙂