HushMail and Countermail do. I think it never really caught on because if you really care about end-to-end security, you wouldn't be using a webmail provider.
But yeah, like you said, if you're only checking emails on machines that are secure and in your control, you might as well just install Tbird/Enigmail. And if you're at the library or a computer lab, you can't trust the client to be secure.
I can imagine an extension for Chrome or Firefox that will not be updated without my knowledge and has been peer reviewed like gnupg.