Wired has a fascinating deep dive into the capabilities and limitations of Hushmail, a Web-based e-mail encryption service that appears to be popular with both security-conscious and criminally minded senders. Seems that Hushmail is not quite as fool-proof as some may have been convinced to believe, especially when determined law enforcement officials come armed with a proper subpoena.
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Voltage Security Network
Though not free, the Voltage Security Network allows you to send ad-hoc secure email to anyone, anytime, without the recipients needing to do anything prior (i.e. get a certificate or a shared secret). They can do this be4cause the service is based on IBE (Identity Based Encryption, a new type of PKI)
However, unlike Hushmail, the Voltage Security Network doesn't store any messages on their systems. Messages are only stored in the sender's Sent folder and the recipient's inbox. The service only manages users' keys.
I don't know of any other encryption service that can send add-hoc secure messages AND not store the messages on the service.
http://vsn.voltage.com