I bought this 8 channel relay module from China for only about $10. http://www.icstation.com/icstation-channel-micro-relay-module-icse014a-p-5185.html
I've also seen identical or similar modules from other vendors including ebay.
Unlike the typical relay module, this is USB / serial driven. So it is possible to drive up to 8 relays using only one or two I/O pins!
Send it 0x50 at startup to get its attention. It will return a byte identifying itself. You can ignore it, but you must send this 0x50 or the module would not respond to anything.
After that, anytime you want to control the relay(s), send it a 0x51 followed by a byte containing the bit pattern corresponding to the 8 relays.