THA Hooks Interface allows you to customize and alter theme functionality from within dashboard by hooking into Theme Hook Alliance hooks.
It seems obvious that a first plugin to fully make use of all Theme Hook Alliance hooks would just give you a bunch of boxes to throw some code into. That said, it's fantastic for what it does, which is give you a way to easily throw content or code into any of those areas of the theme that the THA hooks were designed for.
Want a subscribe link under all your posts? Done. Entry tab, use tha_entry_bottom. Want some sidebar ads? Sure -- Sidebars → tha_sidebars_top / tha_sidebars_bottom. It's a shame there aren't more plugins that use THA hooks, but until there are, this plugins provides a valuable entry point for adding that functionality to your site.