I came across this "trick" today and will try it out. If you use your gmail account to purchase an item at Amazon, I use this email address email@example.com.
Gmail ignores everything between '+' and '@'. I believe that Gmail treats that as a filter. So, if Amazon sells my address to some Pr0p3cia, at least I know who sent it to them.
However, do not use this if your email address is the same for your login identifier. For example, if you try this with your Netflix account, you will find that you can not login as the '+' seems to be stripped out.