do you want to build this yourself, or buy something premade?
for DIY, look into a microcontroller - you can get them with USB capabilities built in, and have dozens to 100s of switches available.
for off the shelf, look for a USB IO board
check this out:
https://www.phidgets.com/index.php
check out their 0/0/4 board ... all it is, is a USB microcontroller, some transistors and four relays
as far as power line control, look into the X10 protocol... again, its something you do with a microcontroller and a zero crossing opto-isolator