iBreakUp

http://www.ibreakup.net/
Use this free service to break up with your soon-to-be-ex signifigant other; it sends an email with the news. You can also choose to make up. Some of the options for letters are pretty funny.
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Email Santa

http://www.emailsanta.com/
Email Santa Claus. Just fill out an easy form (for babies you can enter decimal points) and you get an instant response. Also find out how naughty/nice you are (I got a B+).
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Bloody Finger Mail

http://www.bloodyfingermail.com/
Use the bloody finger to write a message and send it to your friends.
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Pee-Mail

http://www.pee-mail.com/
This little kid drinks a whole bottle of soda and is ready to write your message. Then send your yellow snow message to a friend.
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Scrabulous

http://www.scrabulous.com/
Scrabble online. Play by yourself, with an online group, or challenge a friend over email. Register for free.
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FutureMe

http://www.futureme.org/
Send an email to yourself in the future. You can write whatever you want and set it to be sent as far as 30 years into the future. I read the FAQ and it said your letter will be returned as thepast@futureme.org. You can also go look at public posts.
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