A few Mountain Lion tips 1

Calendar application (former iCal)
If you have Mountain Lion, 10.8, you can get back the mini calendar and the calendar list by clicking ''Calendars'' (wasn't there in Lion, 10.7).

Calendar Alerts in Notification Center (10.8) is configured in Calendar ->  ''Settings'' -> ''Alerts'' where you also can set if they should be used on this computer only.


Contacts application (former Address Book)
You can start mails, iMessages and FaceTime connections from within the Contacts application by clicking the title of a field (e g ''mobile'', ''work'' or ''iPhone'') in Mac OS 10.8.

From the left bottom of a contact card you can share the card via mail, iMessage or AirDrop (10.8).


Reminders application (new in 10.8)
Now you can have location based reminders on the computer as well.

To-do lists can be exported from Reminders application as ICS files which means they can be imported to calenders/calendar applications.


Notes application (new in 10.8)
If you liked applications like Stickies you can double-click a note in Notes application to have it opened in it's own always visible window.


Mail
Apple Mail for 10.8 has a new sorting option, to set certain senders as VIPs. By doing this (clicking their address in a mail) they get marked by a star and get their own Smart Mailbox.

There's also two new options for setting mail alerts and what counts for the number shown at the Apple Mail icon (if you choose to show anything at all). In ''Settings'' -> ''General you can set some options like ''Inbox'' or ''VIPs''.