There are a number of essential plugins if you are running a WordPress site.
Askimet is the span tool that comes with WordPress. There are others available, but I have found that Askimet works just find in most cases.
The next tool you should have is an SEO plugin. All In One SEO was almost an industry standard, but recently more and more people are going over to WordPress SEO by Yoast. I find it easier to use and also the way it analyzes your posts in order to optimize them for the focus keyword is brilliant and very helpful, especially with my clients.
The next essential tool is a plugin that backs up your database automatically. This is essential in case you have server problems. There are many good WordPress plugins that handle database backups. I use to use WP-DBManager, but the creator is no longer full time working on his plugins so I worry about security. So I am actually in the process of testing a few backup tools.
If you have any suggestions on key WordPress Plugins – let me know.