Note this is a third party web site, and its operation and safety cannot be checked.  Use at your own risk, and perhaps change the password soon after registering as a safety measure.

Second Life Registration API: http://modularsystems.sl/regapi.shtml

• They indicate that it gives a list of around 95% of all available last names in one go.
• Note that it also states that the e-mail address used must be unique, a restriction imposed by Linden Labs.