is it possible to connect a tv to the home network via ethernet without putting a computer at each tv?

Answer 1

This is a very dumb idea, ethernet is basically 4 twisted pairs of copper. You won't get very good signal using cat 5, whether or not you get an interface that will even work. Much, much, much easier to simply install coax.

Answer 2

It depends what you want to do with it. There are quite a few "media extenders" around like Apple TV and Netgear's EVA700/800, among others. These are just router-sized boxes, that let you transfer content residing on a PC to the TV or stereo. SlingMedia also has something coming out soon called a SlingCatcher that will supposedly redirect any PC content, including arbitrary streaming media to your TV.