Generally, we're talking about a maximum of 12 volts and maybe 40 milliamps with the Arduino. Not in these projects.
Switching mains power with an Arduino
This may seem incredibly basic, yet if you visit any Arduino forum, you'll find people asking how to pull it off. Their first ideas often seem incredibly unsafe and probably like a fire hazard. However, switching mains power with an Arduino can be very simple, even with a beginner in mind.
All you have to do is buy an appropriately rated relay board. You have several to choose from, most notably the DFRobot Relay shield and the Hacktronics shield pictured above.