sl-user-create

This works great and you can disable regular registration to make a site SL access only! If you have trouble getting it to work, try disabling the signature check and leave "Allowed simulator DNS entries" empty.

It truly requires one click for the user to register, which can create a few accidental registrations on your site. The remedy would be to make it very clear in your objects what they do - but then again, SL being laggy, if you do it with textures, they may not fully load until your visitors are already impatiently clicking on things. Still, minor issue. In modern setup, you need to add the word "resident" to your avatar name to make it work. YourName Resident rather than Your Name or YourName. And it is your user name rather than your display name, naturally. (If there's an error that says you cannot own register objects, check so that your name is spelled exactly as it is in the "allowed avatars" list in your WP dashboard. Secondly, the object won't seem to work straight away. It seemed to work after I clicked "Compile LSL" quite at random not even knowing what it means. It seemed to add it to the tracked objects list. (Use at your own risk. Try refresh scripts first.) (Right-click on the object, click More-More-scripts-compile LSL)