I’m a huge fan of Google’s online applications: GMail, Google Reader, etc. One little known feature of GMail which I find myself using over and over again is the ability to add extra metadata to my gmail email address to let me know where the email came from.
Let’s say that I have a gmail address: email@example.com
Now if I am signing up for a newsletter at XYZ Corp. I can register with the email address firstname.lastname@example.org You can put in any extra alphanumeric information after the plus sign and it still gets delivered to email@example.com. Furthermore you can apply a filter in Gmail to process the email a certain way based on the email address that you signed you for (firstname.lastname@example.org).
One handy use of this feature is to track where emails are coming from so if you started to get spammed for instance you will know which address was passed onto a third party.