Invisible Note is like Chatroulette for e-mail
It's like a game of bouncing e-mail: You send out an e-mail to any address @invisiblenote.com; if anyone else sends that address an e-mail too, they get your note, and you get theirs – along with the e-mail addresses you both sent your notes from!
I tried an obvious one, and it indeed worked. Now a certain Mr. Anderse (yeah, with an E at the end) has my e-mail, and I have his. His message was as plain and uninspiring as mine, obviously meant to just check whether or not the service works.
This is a really fun little game; the only problem is that it's very easy for spam bots to abuse. A spam bot only needs to bomb a bunch of dictionary-word addresses, and get back a list of e-mails all ready for spamming. Some sort of Captcha or other mechanism could really help.
Still, it's a great idea, and if it picks up momentum (and maybe gets a more design-y look), it could definitely be a great way to meet new people!