There are thousands of WordPress Plugins available, but unfortunately a lot of them suck. Why? The most common issues that I find are removing/changing some of the WordPress Core functions and incorrect CSS code that breaks the theme layout.
Here are 6 plugins that are absolutely great (each one doing his own job):
Speed up your website.
Mailing lists, the easy way.
A powerful shop module for your website.
The best Search Engine Optimization tool. Period.
Build all types of forms with ease.
Enable and control multiple languages on your website (paid).