How do you sync your Outlook contacts with Gmail?

You can use the free Google tool Google Apps Sync for Microsoft Outlook :

Google Apps Sync synchronizes nearly all of your personal contact information between Outlook and the Google Apps interface so you can reach colleagues, family, and other contacts from either interface.

You can download it from here.

You may also use file export and import, as described in:
Merge your Outlook and Gmail contacts

OggSync

The basic version is free and it supports features like Outlook's color categories.

KiGoo - Manage Google Calendar and Contacts through the Microsoft Outlook interface.

KiGoo is shareware, try before you buy.