To get to these tweaks, just got to the Labs page (at the top of Gmail, click Settings and then click the Labs tab). Then scroll down to find these two hidden gems:
Don't forget Bob. After you pick a couple of recipients, Gmail will suggest other people to add based on your past email patterns. If you usually CC your boss when you send email to Susan, Gmail will catch on and start suggesting that for you.
Got the wrong Bob? This one is potentially even more useful. It's easy to accidentally enter the wrong person in the TO line if your address book contains multiple people with the same first name. If you're emailing more than two people, then Gmail will confirm if you've chosen the right people that share a first name with someone else in your contacts list.
Here are some other cool Google Labs experiments you might want to check out, too: