O Contacts (Contacts), a native Mountain Lion application before OS X 10.8 it was called Agenda (Address Book), is far from being a powerful repository. There are many other options with more possibilities and integrated with social networks, such as Cobook.
However, it still has its value. If you are well organized / methodical, the app can be a great ally. By clicking on the phone labels (cell, home, work, etc.), a range of options opens, including ?Show in Large Letters? (if you want to make a call), ?Send Message?, ?FaceTime? and, if you have Skype installed, make a call through it.
Options like these appear on almost every label. In particular, I find interesting the options shown in the address ("Map this Address", "Copy Address Label" and "Copy Map URL") and on Twitter, tweeting directly to the contact or viewing the person's tweets.
Another cool tip to customize the ?personal card? template (Model Preferences), choosing exactly what standard information you want to view when adding a new contact. Here I leave options marked as Name, Surname, Surname, Position, Department, Company, Telephone (mobile, home and work), Email (home and work), URL, Birthday, Related Name, AIM, Twitter and Address (home and work) ). There are many, and the cool thing is that you select the ones you think are minimally important to have from each of your contacts, which will help you create a standard and organize your agenda.
Don't forget to also pass the tab General and choose how you want to display and order the names, the address format (Brazil or any other country) and where you want to save contacts by default (on iCloud or only on Mac).