You probably know that you can create multiple email addresses in Gmail adding the plus sign or dots in the username, but below we will see something more interesting.
By creating one account Gmail, Google has two email addresses for you. One uses the regular domain and is @ gmail.com while the second address is under another domain: @ googlemail.com.
Two at the "price" of one.
How can you use the second address?
You can take advantage of these two domains, for less spam in your email inbox.
For example, you can only give @ googlemail.com to your close contacts. The email that gmail.com uses as a domain can be used publicly (put it on your blog, for example).
You can then set a filter in Google email service that will redirect all messages from @ googlemail.com to a special folder. This way you will never miss important emails from your close friends.