This is a truly wonderful application if you're looking to integrate your Google apps (Gmail and Calendar) with your Mac.
While it's easy to integrate Mail with Gmail by using Google's POP3 and SMTP services, the calendar and address book are harder to manage.
This program makes a nice little background program which neatly handles the sync for you. When it starts up (ideally as an automatic startup task) it will keep your iCal calendar of choice in sync with a Gcal counterpart.
For contacts, it cleverly publishes an LDAP server so that all your Gmail contacts are searchable and addressable within the Address Book and Mail. (This is, however, one-way and does not keep them in sync as it does the calendar -- you merely have access to your online calendar from your Mac applications.)
Configuration is fairly involved -- this is not a point-and-click application! But the documentation is solid and the results are nothing short of astonishing.