Thanks to everyone who participated in this topic!
As Discourse has grown from 2013 to 2018, it has become difficult to keep this topic up to date. As of 2017 there are more than 10,000 active Discourse instances on the internet, so we’re no longer sure a manually curated directory of Discourse instances is as useful as it may have been in 2014 when Discourse instances were rare.
We are still thinking of building an automated, general Discourse directory, but for now if you are looking for active examples, we recommend our official customer list.