Looking for simple Ethernet (RJ-45) controlled on/off switch for low-voltage DC line
Good search terms would be "Ethernet Relay" or "Ethernet I/O".

A quick search turned up several possibilities.  An interesting on was from www.relaypros.com, look in the "Wired Relay" -> "Ethernet Interface" category.

 

I am sure there are other possibilities; this approach would be a little less work than creating your own with a microcontroller based board.