You need to decide on a technology to use first, the two most prominent would be:
PGP - good open source support, not supported out of the box by most MUA's
S/MIME - pretty good open source support, supported out of the box my most MUA's

Truth be told, both probably have a place depending on the crowd you're sending your signed emails to.

If you want to use PGP, check out GnuPG (http://www.gnupg.org). You can use Gnupg to generate keys, verify others' keys, and sign/encrypt just about anything, including email. For email support, it depends on your MUA. If you're using Outlook, check out the GData plugin (http://www3.gdata.de/gpg/).....the site is in German, but it's pretty easy to figure out and it free. If you're using one of the major open source MUA's, check out Enigmail (http://enigmail.mozdev.org/). Enigmail works in conjunction with GnuPG to provide PGP mail support.

As far as getting keys/certs, you can generate your own with PGP. If you want to use S/MIME, check out Thawte's freemail community (http://www.thawte.com/html/COMMUNITY/) for getting free certificates.

--Ben