CACert.org is a public Certificate Authority (CA). For non-admin types, this is a self-proclaimed issuer of free SSL certificates.
Is it worth anything? Like a lot of other things on the Internet, the answer is "it depends". It depends on how well people trust the site and use it. Note: You don't have to use Verisign, you can issue your own certificates. Verisign's strength is that, by way of government sponsorship, the majority of users "trust" it as a CA.
Update: For those that are interested in rolling your own, check out the "OpenSSL Certificate Cookbook".
[joatBlog]
Majority of users trust Verisign as a CA because they don't get any scary certificate messages when they browse to a site that is protected using a SSL Cert issued by Verisign. This has more to do with the fact that the major browser vendors have by default included Verisign as a trusted CA.
UPDATE: Just saw Dana's post about this topic. Lot more info and definitely worth a read.
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