Thawte gives away free personal email certificates at their website.
A thawte Personal E-mail Certificate in conjunction with the thawte Web of Trust allows you to secure and guarantee authorship of your e-mail communications by digitally signing and encrypting your e-mails.
IN SHORT: A personal email certificate lets you digitally sign all your outgoing email so that the recipient knows that you sent it!
Click here to get a certificate.
A word of caution here, read everything carefully whilst you apply for digital certificate. Remember the password and the question-answer pairs otherwise you will *never* be able to get another certificate for the same email ID. Also keep your password totally secret – a recipient can take you to court for documents that appear to be digitally signed by you, but was in reality signed in your name by an identity thief!