There are remote-controlled RF switches for $20 for four. The Raspberry Pi can send RF signalss to these switches. Unfortunately, I don't have an oscilloscope to reverse-engineer the signal for mine, so the most I could manage to do is turn them off (but not on).

That's a very safe way of doing these things. If anyone else knows how I can reverse-engineer the signal, I'd appreciate it (I also want to make the Raspberry Pi open my garage door, same deal).