The classic Inferno Shoutbox is now available for free MyBB forums.
Please read the support thread for this plugin for any information you may need.
http://community.mybb.com/thread-149231.html
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This plugin requires the latest development version of Plugin Library (as it includes a bugfix for mybb 1.8), otherwise the CSS may not insert correctly. You can grab that at: https://github.com/frostschutz/MyBB-PluginLibrary The plugin is updated for 1.8, it WILL NOT work on 1.6.
If you encounter errors loading the shoutbox, make sure that all files are readable by the webserver and that /inc/plugins/inferno/anus.php is writable. Also make sure that optimizations such as Rocketloader on Cloudflare are turned off.