c) use a single wildcard certificate or a certificate for more than one site. Basically a single certificate which is valid for both abc.com and def.com. Searching around on Google should reveal a wiki with more information about how to do this and remain compatible with the major web browsers (I think the link should also be in the CentOS list archives).
d) Use a CAcert cert with a AltSubjectName for each domain name. I have posted about this before to this list, this wiki page documents how to do it:
http://wiki.cacert.org/wiki/VhostsApacheThis is not suitable for ecommerce and such like since users need to install the CACert root cert to avoid warnings but for other purposes it's a great solution.
That's not (d), that's a further elaboration of (c) with a link to the wiki I was thinking of, and see it's even in the CentOS List archives...
LOL, Cheers, MaZe.