WordPress now has over 30k plugins in its repository. There are plugins for just about everything you can think of and even then somethings you might not of though of. Covering the spectrum from Ad management to inserting YouTube videos. Specialty plugins for SEO and Form Management.
Finding your way though that labyrinth can be a daunting task. The infographic below is a good start to figuring out which are the best ones to use.
If you would like more ideas on great plugins you can join me and my co-host Marcus Couch on WordPress Plugins A to Z podcast with weekly recommendations.