From the same developer as Mail Act-On, MailTags (Indev, $30) lets you apply tags to messages, to help organize them. However, what's even more interesting to me is that it lets you add notes to messages; add calendar events (such as follow-up reminders) to messages from within Mail; and include tags and other MailTags metadata in searches, smart mailboxes, and rules.
When replying to a message, I generally prefer to have my response appear after a brief snippet of the original message, but Mail expects you to type your response at the top of the message, with the quoted portion beneath. QuoteFix (Robert Klep, free) corrects this uncivilized behavior, putting the insertion point after the quoted text, where it belongs — and it can also remove signatures and empty lines from the original message.
If your server-based spam filter isn't perfect (or if you don't have one at all), SpamSieve (C-Command, $30) is the best tool by far for keeping junk mail out of your Inbox. And on the rare occasion that it makes a mistake, you can train it to improve its accuracy in the future.
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