It is known that “+” sign cab be used to create unlimited virtual gmail addresses using one box! For example you have mailbox email@example.com, all addresses user+ANY_WORD@gmail.com are valid and will be delivered into your mail box. Good usage for this is leave not your direct email on site signups but something like firstname.lastname@example.org, which can be filtered later. For another cool address tweak see sub cat..
It is possible to insert any amount of Dots into username part. Like email@example.com is the same as firstname.lastname@example.org.