WebSecure4Linux is an unofficial, open-source Linux client for Zero-Knowledge Systems' Freedom(r) WebSecure service.
Note that you will need to sign up for the service by obtaining a WebSecure activation code and creating a user account and password before this client will operate. You can sign up at http://www.freedom.net/products/websecure/.
Activation codes are sent to your e-mail address. (Once your account has been created, you might want to skip the client download. Unless, of course, you also run Windows.)
This is not supported by Zero-Knowledge Systems AT ALL; it's completely unofficial. You can try to get support at the SourceForge project page.