Contactizer Pro is designed to help you organize and share all your personal and business contacts and information. This sort of CRM/PIM business software has always lagged behind its PC counterparts, but Contactizer Pro is one of the better choices available for the Mac, with a fairly beefy feature set and a clean, relatively intuitive interface. This app helps you track projects, tasks, and communication (including calls, chats, documents, and faxes, with alarms and follow-ups to invitations), and it also does a good job of integrating with Apple's core apps. During installation, Contactizer Pro syncs up with your Mail, iCal, and any mobile devices via iSync (plus you can sync up your events and tasks on multiple machines with MobileMe). Integration is also solid with Bluetooth devices and Skype, and you can make and log calls from within Contactizer Pro.
As with all complex applications of this kind, there is a bit of a learning curve, especially as you learn this app's particular interface and workflow. Unfortunately, this is exacerbated by Contactizer Pro's biggest drawback: while it has active support forums, there is very little documentation available, which is difficult to understand given the application's price and complexity.