Gmail has long had a feature that automatically suggests and fills out the name of people you've corresponded with. On Tuesday night, the service got a tweak that makes that process easier--it's also likely to be second-nature to Microsoft Outlook users.
Now clicking on the link next to the "to" field pulls up Gmail's contact list manager, where you're able to very quickly sort through your contacts, or anyone you've e-mailed, and pick the ones you want to include in the message. The same goes for removing anyone; you just have to click on their name again and they get removed.
Not readily available when using this new menu is a way to select which of these users you want to add as CC's or BCC's. Outlook does this in the same menu, whereas in Gmail, you have to open up each of those fields in the message, then click to open up the contact manager yet again. Hopefully future versions streamline this process and combine those options into the same UI.
This may seem like a very small feature, but for heavy Gmail users it removes the need to create special lists of contacts they e-mail on a regular basis. Instead, it makes use of regular e-mailing habits and more deeply integrates the short list of people you're communicating with--the same one that's found in Google's contacts manager.