Technically you could plug in whatever you want. MySql, FTP, E-Mail servers... it's up to you. The core application (Etilic Core) handles the connections on various ports bound to various ip addresses (abstracted as virtual servers). Each virtual server has its own set of modules (an http module for instance) and those modules have their own sets of extensions (a web server, for instance, could have extensions for: python, php, c#, etc.).

Whether or not your virtual server is exposed to other systems is entirely up to you. If you bind the server to 127.0.0.1, you can access it from the local machine only. If you bind it to the network ip of your machine, other computers in the network can access it as well.