Where can I find a list of icons used by Discourse?

Discourse uses the free font awesome icons. All available icons that can be used are found here. You may have to add some icons to the svg icon subset in the site settings. You can see what icons are on your site by adding /styleguide/atoms/icons to the URL. If this does not appear please check styleguide enabled in site settings.

If you want to access the pro icons you need to install the Discourse FontAwesome Pro Plugin plugin.