Have Google Help You Remember Email Addresses
Google has introduced a new Lab accessible via Gmail.
“Got the wrong Bob?” keeps track of email addresses you email most often. If you send an email to a group of people that usually includes, say, Bob Smith, but accidentally add Bob Roberts, the Lab will prompt you to make sure you didn’t mean to exclude Bob Smith.
Google has also changed the “Suggest more recipients” Lab name to “Don’t forget Bob.” This Lab is similar to “Got the wrong Bob?” in that it keeps track of groups of addresses you often send emails to, and if you leave someone off the group list, it’ll prompt you to make sure you aren’t leaving somebody out by mistake.
These Labs are accessible in Gmail via Settings > Labs.

As with any Lab, to try it out just click Enable to turn it on, then click Save Changes at either the top or bottom of the window.
For more on “What about Bob?” and “Don’t forget Bob,” see Google’s website.