Although it's not advertised, most mail providers allow you to create custom email addresses and have the mail delivered directly to an existing address. This is handled by the addition of a plus sign (+) in the address.

Say, for example, that you have the address: test@gmail.com. You can use any address of the form test+anything@gmail.com; for example, test+tipstrs@gmail.com. Any mail sent to that address will be forwarded to test@gmail.com. When you get the mail, it will show the full address (test+tipstrs@gmail.com) in the 'To' section... at least, it does in gmail so you know exactly which of your addresses was used.

The great thing about this is that you can use a different email address for every service you sign up for. If you use unique addresses like this, you can easily see where/who is the origin of SPAM, and you can filter this particular address so that you no longer get mail from it.

Several years ago, I found out about a security breach at Expedia in exactly this manner. I started to get a lot of SPAM from my expedia-specific address so I called them up. After much arm-twisting, they admitted that their database had been compromised. I filtered that address, and all that SPAM went away.