A little like a Swiss Army Knife, Jetpack is a useful free plugin with a number of helpful functions (thirty-six in fact). While not all are of equal value, you can activate the ones you need/want. In the words of the creators, Automattic:
“Bring the power of the WordPress.com cloud to your self-hosted WordPress. Jetpack enables you to connect your blog to a WordPress.com account to use the powerful features normally only available to WordPress.com users.” (WordPress.com is, of course, the sister platform to WordPress.org.)
There are a number of features we use on every site, but the one I’d like to draw special mention to is Brute Protect.