Related link: http://www.cacert.org
A couple of my fellow O’Reilly bloggers have already mentioned CACert:
Andy Oram at CAcert: Digital certificates become free
Rob Flickenger at Distributing Your CA to Client Browsers
I’d just like to add that you can use certificates from CACert to sign your VBA macros in Excel, Word, or whatever. It sure beats paying Verisign $400/year!