[joshuak]

GPGMail is nicely integrated into Mail.app. It plays well with s/mime, so you can have both installed, and use either one that is appropriate.

For easy steps on how to do this see here:

http://arstechnica.com/apple/2011/10/secure-your-e-mail-unde....

You can get a free cert from COMODO:

http://www.comodo.com/home/email-security/free-email-certifi....