Go on Ebay and find an old National Instruments DAQ (preferably a DAQmx USB or a bus card) then download Labview. There's a bit of a learning curve but once you've got it it's a super powerful tool. I am actually setting up a new NI DAQ/Labview system as I type this to control my experimental set-up.
I made a really simple array of transistors that drive +5V circuits to control relays. Each of these relays can handle mains voltages. Took me about a day of soldering to set-up. If you want to cheat you can always buy power switch tails but you're paying a premium for something you could build for $5 or less.
I think the Dr. DAQ system is sufficient but if you want a bulletproof, reliable system with HUGE flexibility and capability I'd highly recommend the NI/Labview route. Make sure you design all your relays to have the proper fail safe in case your computer get's shut-off by accident.
I do this junk all day long so if you have questions on any of it feel free to fire them my way...
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