If you haven’t heard of 37 Signals’ Highrise, it’s a web-based CRM tool and is excellent at managing relationships with clients, colleagues, vendors, etc. You can keep track of emails, notes, and tasks for people.
Greatascent is a new plugin for Address Book that lets you synchronize your contacts with Highrise automatically. It also gives you an easy way to enter tasks or notes for a contact without even opening a browser.
It’s in private beta now, but works great from my testing so far. You can register for the private beta by sending an email to Simon, the developer. Visit their website at greatascent.com.