Here's an example of what I'm going on about. Say your email is email@example.com.
Adding +[item] to the end of your email will create an alias that conveys information to firstname.lastname@example.org. For example, when you register on MySpace, you could make your email email@example.com.
All emails to firstname.lastname@example.org will go to email@example.com.
Another neat Gmail feature is you can move the period in your email address. I could give someone firstname.lastname@example.org, or email@example.com. It all goes to the same inbox: firstname.lastname@example.org.
According to the Gmail blog, these features make it simple to set up a filter for different types of emails you receive.
Benj uses aliases (email@example.com) on his email, and it's a good way of finding out who sells your data. Even companies who claim they're not "selling" your information may be giving it away.
People who've (apparently) redistributed his: Microsoft, Google, even grad schools.
When I worked for a company that dealt with schools and districts regularly, I observed the person typically in charge of a huge list of emails is generally a friendly admin who's liberal about passing that info to anybody who claims to have a legit use for it. Most of the time, people do. The yearbook photographer needs to mail promotions and samples to the kids, or the local community college might want to advocate a cheaper alternative to four-year uni. But somewhere down the line, past that first admin, somebody ends up turning a nickel on those names.
And names are cheap. I've bought thousands of household contact addresses for families with kids under 18, in select states, at less than $80 at a time. I'm not saying this to be creepy; I'm saying it because people should be aware that information is fluid. You can try to block the flow, but you really can't stop it. The most ethical institutions always have a naive secretary, an intern or an apathetic third-party vendor who's ready to pass data on. The problem worsens the bigger the institution gets, simply because it's harder to control what people do.
At the very least, this feature -- which is not unique to Gmail, most "serious" email services have it* -- helps make you more aware of what happens to your data after it's left your fingertips.
*Benj insisted I add this because he's not a Gmail user. He found the alias feature for me because, like a lot of pseudo-techies out there, I use Gmail to make my life easier.
Yesterday I was standing over his shoulder and going "OMG, that's so neat how you do that thing with your emails," and then I spend the rest of the day trawling his junk mail folder in shock and awe over the utter TRANSPARENCY of who's-selling-what. And this morning he was totally like, "Here, look what I found, you can do it too."