Two ways to find out who is selling your info.

Gmail has two features which will allow you to monitor who is selling your information. Nearly everyday I find a site that requires registration and it gets annoying. More annoying is revealing my email address to these sites. I used to use multiple addresses but it became annoying to check all of them. So here are two tips for giving out multiple email addresses via Gmail. These addresses can be later filtered if they become too spammy.

Plus Addressing
I've seen this one mentioned alot. Plus addressing allows you to append anything you want to your email address by using a plus sign. So if my name is john@gmail.com, I can sign up to a site with john+somesite@gmail.com. The plus sign is a valid email character. Unfortunately many site registration forms don't allow it. However this is the first thing I try to do every time. If I start getting spam from other companys to john+somesite@gmail.com, then I know who sold me and I can filter that address from going to my inbox.

Dot Addressing
Not as flexible as plus addressing and harder to know which site sold you. However this can be a great way to filter spam. Gmail ignores periods (.) in email addresses. You can't start or end your address with a period and you can't have two in a row, but you can have as many as you want. The best part about this method is I have yet to find a site that won't accept a period in an address. So jo.hn@gmail.com is valid, jo.h.n@gmail.com is valid as well. Obviously longer email addresses have more choice. You are limited here but I would suggest grouping sites by level of trust. So some site you randomly visit that you feel you must sign up for to post a comment and you know you will never be back, put the period in the first position, j.ohn@gmail.com. A site you plan to use all the time would perhaps get a period in the last position. Or whatever you decide. If you have a 6 character email address you have 31 alternates available. An 8 character email address has 256, but then with so many choices you will need a way to keep track. I would just use three or four at a time and change them as they start getting abused.

Both of these schemes work out of the box with gmail, though the dot technique does not work for gmail for your domain addresses. There is no pre-registering the addresses you plan to use. Go ahead, mail something to yourself at some random plus address. Keep in mind there is a maximum length to an email address, so don't go crazy. Other email providers allow plus addressing or other choices as well. You can get more specifics here: http://en.wikipedia.org/wiki/E-mail_address