Mail
The new version of Mail found in Panther allows to display messages organized by threads
( View > Organize by Threads ). This feature is interesting for browsing more easily mailboxes
with messages from mailing lists, and is as well useful to get messages and their replies all together.
New Smart Addresses in Mail does quite a good job for you : from a person's name, you have
options to use different mail addresses (if the person has more than one and is included in your Address Book),
to add the person to your Address Book or to view the person's card in Address Book, among others.
This makes Mail messages look cleaner as well, showing only peoples' names.