I am registered with a domain name registrar which does not offer an anonymous whois service. However, I am interested in using some third-party service to keep my whois information private. Are there any such services available that do not also require me to register my domain name with them? All I am interested in doing is changing the whois information with my current registrar to point to some company which is able to forward any correspondence to me, while keeping my personal address anonymous.
You could try http://www.whoisguard.com/ -- they say they offer whois privacy protection for 'existing domains' (i.e. without having to transfer them to a different registrar), which sounds like what you're after.
If it were me, though, I'd just transfer the domain to someone who offers whois protection for free as part of the service (like namecheap), or someone who offers it very cheap (like dynadot), but I appreciate that switching registrars isn't always ideal.