The guys over at http://plugintable.com/ think WordPress has become a science and have created a Periodic Table of WordPress Plugins.
WordPress started life as a humble blogging tool and over the years has grown into a very popular Content Management System (CMS). Some WordPress developers would say that developing for WordPress has become a bit of a science (although a relatively easy and interesting one to learn!).
WordPress has had over 23,000 plugins submitted to its plugin directory. Some very good, some not so good. The Periodic Table of WordPress Plugins details the most popular plugins by download in a very easy on the eye way.
Be careful though, the most popular plugins by download do not always mean they are the best. Some of the most downloaded plugins have been around for sometime and although they may regularly be maintained better options may now be available.
When you need a plugin for a WordPress website ensure you search Google for reviews, this should help you make an informed choice.