In my opinion, the best addition to Mail.app in Leopard are data detectors. This feature means that Mail will automatically pick out dates, times and addresses so you can add events to iCal or view a map with just a couple of clicks. For iCal events, Mail even recognises relative dates like "tomorrow" and automatically enters this information when creating a new event.

To use the data detectors feature, simple hold your mouse over a date or time in an email message. A dashed box should appear around the item, with an arrow that you can click to bring up a drop-down menu. The two options available are "Create New iCal event" and "Show This Date in iCal". If you create a new event, a dialog box just like the one in iCal pops up, ready filled with the information. This all happens within Mail, so there is no need to even open iCal.

While this is a great timesaving trick, unfortunately the Mail dialog lacks some of the features that the iCal one has. You can only add a single alarm, and you can't add attendees or attachments. However, it is still possible to use data detectors to fill out your event information and have access to these more advanced options.

Simply hold down the Option (alt) key when clicking on the data detector drop-down arrow. The menu item should now read "Create New Event in iCal". Selecting this will load up iCal and create a new event with the information in the Mail message.

If you already started creating an event in Mail, and then decide you want to add multiple alarms or attendees, simple hold down the Option (alt) key. The "Add to iCal" button at the bottom should change to "Edit in iCal", allowing you to make the changes you want.

Enable Data Detectors in iChat
Apple also built data detector functionality into iChat, but chose to disable it for some reason. Luckily, enabling it again can be done by a simple Terminal command. Start by opening up Terminal (/Applications/Utilities). In the window that appears, paste the following line, and hit enter.

defaults write com.apple.iChat EnableDataDetectors 1

Next time you open iChat, data detectors should be enabled. To disable them again, repeat the command with a 0 at the end instead of a 1.

Disable Data Detectors in Mail
If you never use data detectors, you might want to disable them. This can be done using the following Terminal command:

defaults write com.apple.mail DisableDataDetectors YES

Next time you open Mail, data detectors shouldn't appear. If you want them back, just repeat the command with NO at the end.