If your Gmail login name was email@example.com and you went to samplesite.com to fill out a registration form, instead of just entering firstname.lastname@example.org as your email, enter it email@example.com instead. When Gmail sees a "+" in an email address, it uses all the characters to the left of the plus sign to know who to send it to. In this example it would still send it to firstname.lastname@example.org.
Now whats cool is if you search Gmail for username+samplesitecom, you will see all massages that were sent to that email address.
To see who is responsible for sending a specific message click the Show Details link and you will see the complete address"
copied from diTii.com