I made this plugin for my forum to promote registration and member activity and thought I would share. It allows guests to read up to 75 scrolls or "ticks" of a thread then locks the site up asking them to either login or register to gain full access.

Install in inc/plugins then enable in the ACP.

You can style the message box anyway you would like.

This isn't attended to be a security plugin, do not use to give previews to paid content. It can be bypassed with little web development knowledge.