For 35 euro I purchased this gadget ("Mediamarkt") and I am quite happy with it. I can send wireless internet to my Samsung TV now. Most of the times I use it to watch YouTube or websites via the Google’s Chrome browser (using the tab casting feature).
You must have access to a wifi network. It is not possible to connect the Chromecast dongle to a LAN-connector.
It is funny that you can cast video via your smartphone as well when you have the Chromecast app installed. The development of lots of apps that can work with Chromecast is on going.
I think it is a smart, inexpensive and usefull gadget.