On the add and edit contact pages, you'll now see a "Custom" option under all label list boxes, for all the contact fields: email, phone, address, web site, instant messaging and date. Select this option, and input any value for the label.
With this new feature, you're no longer limited to a predefined set of values for the labels, like "Work" and "Home". You can input any value for the label, for example: "Bob's assistant" for a phone number:
The custom labels will sync with your Google contacts.