We made a list of some useful plugins for Magento 1.9.X that we have used on client’s projects.

1. Transactional STMP Mail Plugin

Essential if you use a 3rd party service for all your transactional emails, like Sendgrid: http://www.aschroder.com/product/smtppro-magento-smtp-email/

2. Automatically Refresh Invalidated Caches

Magento 1.X has a habit of not refreshing invalidated caches automatically, for instance after you update product categories, block html, etc. This extension automatically checks and fixes invalidated caches: https://github.com/tomasinchoo/Inchoo_InvalidatedBlockCacheFix

3. Wordpress Integration for Magento 1.9 and Magento 2.X

The free version of this plugin provides the basic functionality to integrate a wordpress blog inside your Magento site. However, most Wordpress plugins and things like shortcodes will not work until you purchase additional plugins from the developer: https://fishpig.co.uk/magento/wordpress-integration/