Such "splitters" are always called routers and switched hubs. The router costing about as much as some switched hubs with the advantage I could make that work everytime.
I will be visiting a friend for a little while and I need to have access to internet on my laptop. However, my friend's internet is set up that his computer is connected to the internet through an ethernet cable going into his vonage modem (which doesnt have any extra spots) which goes into his regular modem. He doesn't want me to put in a regular or wireless router. So, is there anyway for me to plug my laptop into the internet (besides disconnecting his computer while i use it)? Do they make something like an Ethernet cable splitter that I can plug both computers into and then plug that into his modem? Thanks for any advice!