Akismet – Spam fighting, requires a free Akismet account to get a key for it to work
Cforms – Form processor, not open source
Ninja Forms – Form processor, open source
Events Manager – calendar of events, registration for events, creates many widgets to use to publicize events
Wordpress Editorial Calendar – way to plan out blog posts over a calendar interface
Google Analyticator – way to include Google Analytics in your blog
Google Analytics for WordPress – way to include Google Analytics in your blog
WP-Stats – non-Google statistics plugin
Image Widget – gives you an easy to use image widget that takes an image and a link, does not require HTML coding to work
Jetpack – Large plugin that does a number of useful things
Go to Plugins –> Add New and enter any of these plugins into the search bar, find the right one and click install. Some (like Akismet) may require additional configuration.