Earlier this year, I , that provided preliminary Leopard compatibility. But the developer, Indev Software, has recently released Mail Act-On 2.0.1, and this major upgrade warrants a full review, as it adds many new features and makes Mail Act-On a required add-on for many Mac users.
Like the original version, Mail Act-On 2 lets you apply specific rules to selected messages at any time--in other words, only the rule(s) you want to apply, whenever you want to apply them--via simple keyboard shortcuts. After selecting one or more messages in Mail, bringing up Mail Act-On's rules menu (accessed via F2, by default) shows any rules you've created. Press the shortcut key for the desired rule (or click on the rule name with your mouse, or use the arrow and return keys to choose the rule), and the rule is applied to the selected messages. Because these are rules, their actions apply only to those messages that fit the particular criteria you've defined.
Once you've used Mail Act-On for more than a few minutes, this process becomes second nature. And you don't need complex rules to get good use out of Mail Act-On; I regularly use it just to move messages to particular folders (mailboxes in Mail parlance). For example, I have a rule for moving email to my Family folder; whenever I receive a family-related message I want to keep, I quickly invoke that rule and the message is moved to the Family folder without me ever touching the mouse.
You can also apply any rule without having to bring up the Mail Act-On menu by adding the control key to the rule's shortcut. And if you regularly apply multiple rules to messages, a new feature in version 2 lets you lock the menu window so that it stays open, allowing you to quickly apply rules in succession. Another welcome new feature is the capability to undo rule actions; if you realize you've applied the wrong rule to a message, just choose Mail's Edit: Undo command to reset the affected messages back to their original states (and, if a rule moved them, to their original locations). Unfortunately, in my testing the Undo command was unavailable when I moved mail from an IMAP (remote) folder to a mail folder on my computer; the developer is looking into this issue.
Among heavy users of email, perhaps the most popular new feature of Mail Act--and one that gives Mail a bit more parity with Microsoft's Entourage--is Outbox Rules, which allow you to apply rules to messages as you send them. For example, if you use Mail for both work and personal email, Mail Act-On can automatically file any outgoing email sent from your work account into a Work Sent folder.