How to trigger a relay through a USB port?
You just need a solid-state relay (SSR). Choose one that can be activated with an input of 5V and can control ac power at the voltage of your local mains. Add up the current of all of the devices you want to control, and make sure that the selected SSR can handle that much current on its ac controlled side. The rest is just wires to connect everything.

Be advised that you may need to provide a heatsink for the SSR.