Check your other mail accounts with Gmail
"Now Gmail can check for the mail you receive at your other email accounts. You can retrieve your mail (new and old) from up to five other email accounts and have them all in Gmail. Then you can even create a customized 'From:' address, which lets you send messages from Gmail, but have them look like they were sent from another one of your email accounts. Please note that you can only retrieve mail from accounts that have POP3 access enabled." -- Gmail "What's New" page
The idea isn't new, Yahoo offers the service for their mail application and Hotmail used to offer it years ago, but has since removed that functionality. Google is relatively late to the game, but judging by the response, people don't care. If you have the feature enabled already, let us know how it works by leaving a comment.
[Thanks Corsin, Nate]