Bonjour

Bonjour is a product issued by Apple, widely known as a zero-configuration networking utility. It bundles a group of technologies that are able to discover services inside a LAN, allowing users to set up a network without any configuration.
Initially called Rendezvous, Bonjour is a built-in feature of Apple OS X and iOS devices, but made available for Windows as well through a lightweight and easy to install package.
The components that Bonjour relies on in order to achieve its purpose are service discovery, hostname resolution and address assignment. Using them, the program is able to locate printers, and even other computers, as well as the services that these devices can offer.
Moreover, it enables the possibility for devices to discover each other, without requiring users to perform complicated configurations for IP addresses or DNS servers.
Because of its zero-configuration attribute, a wide array of third-party applications use it in order to facilitate easy access to various system services. The most obvious example is iTunes that uses it in order to find shared music on computers.
Other software that make use of Bonjour are Adobe System Creative Suite, Skype, Pidgin and Trillian, to name just a few. Its popularity is due to the ZeroConf technology, which greatly decreases developer efforts in what service discovery is concerned.
Of course, applications that have been implemented on top of it require that you have Bonjour installed in order to deploy them. But this poses no discomfort, because of the easy and complication free installation process.
In addition, Bonjour itself requires no effort. Simply install it and proceed with your work, as user intervention is not asked for. However, it runs at startup, by default and can be disabled from the Task Manager or Windows Services.
All in all, Bonjour meets the needs of both developers and users, sniffing and discovering services that otherwise would be a challenge to detect and configure.