I've been looking around for ways to turn AC loads on and off. A nicely packaged solution is the XBee Smart Plug - plugs into the wall and you plug devices into it. There are Z-Wave switches which are similar. Still, not everyone wants a wireless solution. What about a good old fashioned wire controlled AC on/off switch?
Looking around on EBay and Amazon, there are mechanical relay based solutions. Searching for "Arduino Relay" was a good start. Generally, all the relays I found switched at Arduino logic levels, and there were a bunch of choices - single relays, gangs of 1,2,4,6,16 relays, with power supplies of 5V, 12V, and 24V. Depending on the board, the relays were capable of switching up to 10A at 240 volts. That's the good news. The bad news is that while most of the solutions had relays rated and approved to switch my AC devices (look for UL and CE marks), the boards themselves did not. Since these boards were not approved, one has to assume that their makers know what they are doing (not always a good idea). So you are taking a chance. If you are using the relays to switch low voltages, all are probably fine.
I had better luck when looking at solid state relays. There's a standard roughly 2" by 2.5" by 1" package with 4 screw terminals which is multiply sourced (look for "Opto 22 120D10", for an example). Many of these relays are UL approved. Unlike the mechanical relays, these solid state relays do not require an additional power supply (they borrow from the AC). They switch at roughly the same logic levels. Unlike the mechanical relays, they have a noticeable, small "on" resistance, so they may require a heat sink.
One of the coolest looking solutions I found was the Remcon Relay Switch. It is also a solid state relay. These are tailor made for fitting into switch boxes and have built in thermal shutdown, if they overheat. They also have the UL approval marking. Unlike the other solid state relays, these use a dry contact to turn themselves on and off, so you need some kind of low voltage relay to control them. These are the priciest relays of the bunch, but they seem like a tough, safe solution.