When composing messages, you probably rely pretty heavily on auto-complete to add recipients. Auto-complete is convenient and fast, and usually does the trick. But sometimes seeing your list of contacts can help you remember all the people you want to include on your email. So, we've added a contact chooser to Gmail. Click the 'To:' link (or Cc:/Bcc:) when composing a message and you'll see something like this:
You can click on the contacts you want to add or search for others.
If you use contact groups, your groups will appear in a drop-down menu in the contact chooser, so you can select contacts from the groups you've already created. And if you happen to use Gmail in Chinese, Japanese or Korean, being able to pick from your list of contacts should be particularly useful since auto-complete doesn't offer the same search as you type experience that it does in other languages.