Gmail does support the "+" separator, although (as I pointed out in another comment) many sites either a) don't allow "+" as a valid character in e-mail address

Gmail does support the "+" separator, although (as I pointed out in another comment) many sites either a) don't allow "+" as a valid character in e-mail addresses or b) simply strip off the "+foo" portion. For example, if your e-mail address is "me@gmail.com", you can send mail to "me+foo@gmail.com" and it will be delivered to you. This makes filtering mail from specific sites pretty easy if there isn't already an easy way to do so for that site.