I actually have one somewhere in my junk in storage. It was an analog modem mated with a hub pretty much. You would connect your devices with Ethernet and the modem would hold it's own dialer setup. When something wanted to connect the modem would call in the provider and it (and any other devices) were online.
In practice it didn't work that good, because once a computer thinks it has an always-on network connection it tends to try making remote connections for all sorts of things unless you lock it down. So the modem was always connected pretty much. This isn't practical when you don't have a dedicated phone line for it and even then many ISPs had systems set up to not allow you to keep connections on constantly like that.