Question: I want to set up a bunch of raspberry pies that each run a webserver, and then allow people to simply go to a website on their laptop and get an overview of all of these raspberries on the network (and then of course they'll be able to do stuff with them by calling the web server).

Is it possible to set up the raspberry pies in such a way that they are discoverable on the local network from JavaScript? Without the need of a server? Or is this only possible from native apps?

Answer: 
* Allow devices an access to your local network (Just Google OSX local network port, Raspberian network access)
* Then use any discovery library that you want.

I'd suggest using something like [Cote](https://github.com/dashersw/cote-workshop) NodeJS (easly runs on raspbery and any device) - it discovers any other process running cote on local network automaticly;

You can also build your own discovery script or just configure in advance
You could install node.js on your raspberry pies and install socketio on your pies. Then you could simply interactive with them from the client side