Possibly a print server - very probably nothing you can use with the Amiga. Most devices with ethernet and serial ports are designed to connect a serial device (=slave) to a network, ie. enable something else to use the network to access the port.
For the other way around, you'll need something completely different - if a device of this kind exists at all. (Well, you could use a COM server to make e.g. a PC connect to the Amiga's serial port with PPP over an ethernet connection - but then again, you can also connect the PC directly.)
Don't forget that if you do manage to connect something ethernet-ish to the serial port, you also need a stack to work with it. If you lack a driver to make an existing stack work, you're bust.
The only thing that could really do the job would be a router with a serial port talking PPP - if it allows you to set the default gateway on the ethernet side, chances are very good to get it to work.
