I am getting a code signing certificate for my open source projects. I have a couple of questions about them:
Certum (http://www.certum.pl) offers free code certificates for open source projects. I know the TortoiseSVN and AnkhSVN projects use certificates from them for their distributions.
The problems with signatures expiring when the certicate does is not specific to a certificate provider but on how you sign the certificate. To keep the signature valid you should also sign a timestamp. See the FAQ of your certificate provider.