It is known that “+” sign cab be used to create unlimited virtual gmail addresses using one box! For example you have mailbox firstname.lastname@example.org, 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 email@example.com, 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 firstname.lastname@example.org is the same as email@example.com.