Below is a list of my favourite free WordPress plugins.
Asset CleanUp: Page Speed Booster
Coming Soon Page & Maintenance Mode by SeedProd
Google Analytics Dashboard Plugin for WordPress by MonsterInsights
iThemes Security (formerly Better WP Security)
MailPoet – emails and newsletters in WordPress
Smash Balloon Social Photo Feed
Smush Image Optimization, Compression, and Lazy Load
Wordfence Security – Firewall & Malware Scan