This may seem like a boring subject, but these little tricks can actually provide some nice little benefits that you may not be aware of.
The dots don't matter
If you're email address is firstname.lastname@example.org you could use email@example.com or any variety of dots in the handle of your address. They simply don't matter. There are some services, like Twitter that do not allow dots in the username, but Gmail does not care.
That one was light, the next one might surprise you.
You can add a + and what follows does not matter
We probably think that our email address has to be exact, but that does not appear to be true. Gmail allows you to add a + then anything you want @gmail.com and it will still be delivered to you.
For example, if your address is firstname.lastname@example.org you can do email@example.com and messages sent to this address will still be delivered to firstname.lastname@example.org.
Ok, so who cares?
It may seem trivial, but let's say you want to setup a fake Twitter account. You are required to have a unique email address in order to set that up. Now you can make an address on the fly. Let's say you have your address: email@example.com. He probably wants to use that as his primary email address for his Twitter account. But let's say he wanted to setup a Twitter account for his business, Basecamp. He could (and realistically probably would) set up a new email account and then setup a Twitter account. But if he wanted, he could go to Twitter, click to create a new account and when he was asked for email, he could enter firstname.lastname@example.org. This would still go to his primary inbox, and it would allow him to create a new Twitter account.
But that's not all
Think about other ways you can apply this. Let's say you are signing up for a new service. If you entered a unique email address for each, it would make it even harder for someone to take your credentials from one service and use them to login to another. Granted you shouldn't use the same password for them, but in case you did, you could at least have a different email address.
Now, that becomes interesting when a company starts selling your email address to third party companies. You could actually see who sold your email address if they don't scan their address list for this. And then you know exactly who to be pissed at.
This would also let you easily setup filters in your inbox, or block spam messages.