Apple's Mail application by itself is not the most fully featured of email tools, it lacks proper calendar integration, column views and more technical things like always connected IMAP IDLE mode. Fortunately it makes up for these shortcomings in its interface, great search functionality and integration with the rest of OSX. But still its nice to have the functionality that is available within contemporary applications like Thunderbird and Outlook and that is where Bundles come in.
Bundles are Apple Mail-speak for functionality extensions similar to Firefox's. Unlike Thunderbird which has the ability to be extended but has so many inbuilt features its almost pointless, Mail has a wide array of Bundles that provide nearly all of the functionality missing within the base application.
Two of my 'must have' free Bundles are Letterbox and IMAP-IDLE which together add column views and idle support for IMAP respectively. The Growl notification framework is also worth installing if you would like visual notifications of incoming mail including sender details and a brief teaser. This is useful as it lets you can decide if it is necessary to read the complete message immediately or whether it can wait till later. Even better Growl is not limited to Mail with a host of plug-ins available for a variety of different applications.
Another very interesting but more costly (US$25-$30) Bundle worth exploring is MailTag 2.0 Beta. MailTag 2.0 adds searchable tagging functionality within email headers and a host of other features that integrate Mail with iCal. The original MailTag stored tag data in files separate to the email repository but with 2.0 this data is stored within the email header. This means if you are using an IMAP server the time and effort placed into tagging your data on one computer can be easily shared amongst multiple desktops. Many of the extra features that come with MailTags look as if they will make their way into the next release of Mail in OSX10.5 Leopard, but as it stands its tagging capabilities will be a unique feature for a long time to come.