Did you know the periods in your gmail address don't mean a thing? That's right, if you signed up as email@example.com, you'd still get emails sent to firstname.lastname@example.org. You'll even get emails sent to email@example.com, for the wiseacres in your family. Ooh, and anything sent to "googlemail" instead of "gmail." Honestly, it's the little things that blow my mind (and btw, none of the above email addresses belong to us!).
More usefully, you can add a plus sign to the end of your address, and then you can set up folders in your email that coordinate with that. For instance, say you open an online account with WellsFargo. When you sign up, give your email address as "firstname.lastname@example.org", then in Settings, set up a filter. In the From box, add in WellsFargo or your chosen bank, in the To box, put in your newly-created email@example.com address, and the set the filter to put in a folder titled "banking." You can also use this technique when you sign up for something and send their emails to you directly to the trash!