Google adds further capabilities to the Chromecast HDMI dongle with a Ethernet Adapter-- essentially a means for customers to "create a fast and reliable connection" for the device via RJ45 cable.
The Chromecast uses wifi for network connectivity, bringing about media streaming problems when a strong wireless signal is not available. One can obviously buy devices to strengthen or repeat wifi signals, but the Ethernet Adapter makes such woes a bit easier to resolve.
The accessory requires no setup, since one simply needs to plug the appropriate cable into the Chromecast before plugging it directly into a power socket.
The Chromecast Ethernet Adapter is currently only available in the US, but images of a British version of the accessory suggests European editions should hit the market soon enough.