Dione Magic Calendar is free highly configurable Joomla! plugin that adds calendar functionality to your pages.

You can customize the date format and language, restrict the selectable date ranges and add in buttons and other navigation options easily.

jFinalizer is to replace tons of JS and CSS files with a single CSS and JS file. This is done by processing the final joomla output HTML. Additionally, you can compact & compress a few things and make your HTML look more compact. for joomla 1.5.10+, PHP5.

Custom Meta Tags is a powerful Joomla! SEO plugin that you'll be able to define custom metadata for each of your articles with HTML title for the serach engines, you can use the metadata configuration of Joomla! or this plugin, the only difference is that, for example and canonical URL, meta keywords.

This allows OpenX ADs to be delivered directly into the top (left or right) of Joomla content (no need to use tag). Use OpenX AD system to control how and when ADs appear (contextual, URL based) in concurrence with the plug-in parameters you set.