Since I solved a number of the issues plaguing my Mail account setup - including a workaround to kinda use Gmail over IMAP (more on that soon) - I have sunk my teeth back into Apple's excellent email client. Now I'm pretty happy with using a few plug-ins like Scott Morrison's stellar MailTags 2 and Stefan Schüßler's Mail.appetizer (which we have previously mentioned at length here and here), though I am continually surprised by the number of plug-ins that endow Mail with everything from better support for Windows Office attachments, to creating events from messages and even more fine-toothed Smart Mailboxes for your Address Book contacts. One only needs to run a search for Mail at MacUpdate to see what I'm talking about, but I figured I could round up a few of these plug-ins just to give you an idea. Mailboxer is an interesting plug-in that can create Smart Mailboxes either for every contact in your Address Book, or only for those you add to a group you specify. This could become really powerful if you combine some of Merlin Mann's Address Book kung-fu tips we mentioned. Mailboxer is donationware and available from earthlingsoft.
Mail unread menu is a customizable menubar item that displays an unread count for all your mailboxes and folders - not just your Inbox. It allows you to include/exclude any mailboxes or folders that you want, and offers a drop-down menu with a list of each mailbox/folder's unread count so you can check exactly what you need without bringing Mail to the front. While you unfortunately still need Mail running in order for Mail unread menu to work, it can update instantly and automatically in the background, ensuring 100% accurate counts of all your mailboxes. Mail unread menu is donationware and available from logan rockmore design (on a side note, this developer also makes a NetNewsWire unread menu item as well, and uses TUAW in a demo screenshot. I tip my hat to you, Mr. Rockmore).
Event Maker allows you to create iCal events and todos from Mail messages or simply from scratch. As more users are looking for better integration between Mail and iCal, this is an increasingly popular space for apps like this and Event Maker - even while in beta - is often mentioned as a leader. It is currently in "heavy" development and free while in beta, though MacUpdate lists its price as $9.95; I am assuming that will be its commercial price once it hits v1.0. Event Maker is available from mamooba.
OMiC should elicit a sigh of relief from the Mail users in the crowd who live in a Windows Office world, as it allows Mail to properly digest those 'winmail.dat' attachments Office users send that are actually bundles of appointments, contacts and/or other Office files. Its developer, restroot.org, offers a brief video demonstration of what exactly you see before and after installing OMiC, which should quickly convince its target Mail users as to whether the €10.00 price (about $13.50 USD as of this writing) is worth it.
If you're looking to expand Mail's abilities, these are a decent place to start, but by no means the end of the road. Run searches at sites like MacUpdate, HawkWings.net and VersionTracker to get a better look at all the features 3rd parties are adding to Apple's email client.