What is a Code-Signing Cert?
Code signing is a convenient way to authenticate the person who's written some code (for example, java code in a .jar file)
A code signing cert has special abilities to be accepted by Microsoft, Sun, Mozilla as code signing certificate.
But anyway, it will working nicely only if the root cert is trusted *and* if you accept to execute the code!
joke part: At least, you will know the name of the person just before you let him/her erasing all your hard disk content 
What do I need to get a code-signing cert?
For code signing you need to have at least 100 points, and CAcert has to have a (scanned) copy of your photo ID. Then you have to send an email to support #at# cacert.org to ask to activate Code-Signing for your account.
How can I use my Code-signing Cert?
Linux
[http://www.hick.org/code/skape/elfsign/] [http://sourceforge.net/projects/disec] (uses GPG keys) [http://blogs.sun.com/roller/page/darren/Weblog/signed_solaris_10_binaries]