Dots and Plus Signs in Gmail

I think this is a very useful information for me, so I just want to keep the article from PC Magazine in here.

An Unofficial User's Guide to Gmail

Think you know everything there is to know about the popular Web-based e-mail application from Google? Think again.

Dots and Plus Signs in the Address
When Gmail receives messages, it ignores extra periods inserted in the first part of the address. That means youraddress@gmail.com is the same as your.address@gmail.com and even y.o.u.r.address@gmail.com and your......address@gmail.com.
What's more, you can append words to the Gmail address, like so: youraddress+comics@gmail.com or youraddress+news@gmail.com. Combine dots and pluses all you want: Messages sent to your.addr.ess+superstar@gmail.com will get there nonetheless.

Why are these infinitely customizable aliases useful? They make it easier to filter messages as they arrive. Give one kind of address to friends, another to coworkers. Append +spam when you sign up for a service you fear might pass your name along, then filter out any messages sent to that address.

Better yet, sign up and append the name of the service you're using (like youraddress+pcmag@gmail.com)—you can tell when it gets passed along. That makes it easy to remember when you have to sign in using your address as a username.
Of course, not all services will let you put a plus sign in your e-mail address, so this technique may not work for you.

One more alias you can use: the domain names of gmail.com and googlemail.com are interchangeable. youraddress@googlemail.com is the same to Gmail as all the above examples. The same alias rules above apply, therefore.