Do you use Gmail? Great, keep reading (and if you're not, go register and come back, we'll still be here). One of Gmail's many cool tricks is the ability to find out which sites have been sharing/selling your email address to third-parties.

The next time you register for a website, add that site's name to your Gmail account by using the "+" symbol (no quotes). For example, if your email address is BurnedInRocks@gmail.com and you're registering an account at ShadySales.com (that's your first mistake, homeslice), use BurnedInRocks+ShadySales@gmail.com. Any emails sent to that addy will still show up in your regular Inbox, but it will show the custom designation allowing you to see which sites are playing shenanigans with your email.
Not only that, custom addresses makes managing junk email a little bit easier. Just set up a rule to automatically torch any email that comes to that custom addy and you'll never be bothered by special announcements or third-parties again. Here's how:
Easy cheesy, right?