Though admittedly, exactly why one might want to, is a little puzzling...
You can write an email, and have it sent at any time in the future..
I guess there is no guarantee that the services will be around in the long term, but certainly things they are working now..
The first website you can use for this is: http://www.futureme.org/
And the 2nd, which will require registration and log in:
Cat painted upon the hand of the artist
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