Ask HN: What are your favourite free and open source Wordpress plug-ins?

I know Wordpress is a bit looked down on in HN circles, but there’s still nothing like it for building advanced websites for semi-pro developers like me.

However most functionality is provided by freemium plug-ins that often try to do way too much to justify the subscription.

I’m trying to build out a list of high quality open source plug-ins that provide essential functionality, as a contrast to the standard commercial model.

So HN, what are the free/libre Wordpress plug-ins that you can’t live without?