A number of the sponsoring orgs will grant you access to their zonefiles if you're willing to do a little paperwork (HelloFax is awesome) and have a valid reason for access. Granted it's not the complete list of registered domains (those without nameservers on file won't show up), but it's pretty close.

The list of sponsoring orgs for each TLD is at: http://www.iana.org/domains/root/db