Yet another separated tool. The Elasticsearch ecosystem is not going to became simpler with ES 5 removing the "site" plugins and everyone doing their own apps instead of building Kibana Apps (i.e. Cerebro, the new Kopf)...
Elasticsearch's plugin ecosystem has historically been fragmented. That's why we make the plugin available in multiple ways (hosted app, chrome extension and site plugin).
well-behaved site plugins (such as kopf) have always been able to run as a standalone app in the browser - either from a folder on your disk of from a stupid webserver. All it needed was a working CORS config on the ES cluster.
It's always been a good idea to deploy them in that way - no need to have the plugin on each node, easier version upgrades, access to multiple clusters etc. So basically, not much changed.
Links:
[1] Hosted app - http://appbaseio.github.io/mirage/
[2] Chrome extension - https://chrome.google.com/webstore/detail/mirage/dcnlpfmnpog...
[3] Site Plugin for Elasticsearch v2.x - plugin install appbaseio/mirage