A smart plug sits between the device that you want to control and the wall socket. Then, from the app you can toggle power to any device that is connected to the smart plug. There are multiple types of Smart power plugs and outlets available in the market.
What types of Smart Plugs and outlets are available? 
Most of the Smart Plugs and outlets available are Wi-Fi controlled and integrate with your existing Wi-Fi network. There are other types of Smart Plugs which require a separate control hub  since they run different type of communication protocol different from Wi-Fi. You should ideally avoid them unless you lot of outlets and other items such as dimmers, switches, lights, fans from same manufacturer.
Do Smart Plugs have Voice Control?
Most of the plugs have their own app, but it is important to choose Smart plugs and outlets which have both Amazon Echo/Alexa and Google Home integration or atleast one of them which is preferred by you.
 Do smart plugs replace existing switches?
Some of the smart plugs we tested were completely integrated into the wall socket (they replaced wall sockets) but they were difficult to install and may need professional help.
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Manufacturer
Model
Amazon Link Rating Voice
Control
App
Support
Energy
Monitoring
Unique Features
Belkin
Wemo Insight Smart Plug
Read detailed review here
Amazon Alexa
Google Home
Apple HomeKit
Away mode:
Turn on/off Lights when you are away from home
Belkin
Wemo Mini Smart Plug
Amazon Alexa
Google Home
Apple HomeKit
iHome
iSP8 Wi-FI SmartPlug
Amazon Alexa
Google Home
Apple HomeKit
Handheld Remote
TP-Link
Kasa Smart Wi-Fi Plug Mini HS105
Amazon Alexa
Google Home
Apple HomeKit
Away Mode:
Schedule the Smart Plug to automatically switch on and off when away
TP-Link
Kasa Smart Wi-Fi Plug Mini HS110
Amazon Alexa
Google Home
Apple HomeKit
Away Mode:
Schedule the Smart Plug to automatically switch on and off when away
Insteon
Smart Plug
Amazon Alexa
Google Home
Apple HomeKit
requires insteon hub
Etekcity Voltson
Wi-Fi Smart Plug
Amazon Alexa
Google Home
Apple HomeKit
iDevices
Switch WiFi Smart Plug
Amazon Alexa
Google Home
Apple HomeKit
iDevices
Outdoor Switch WiFi Smart Plug
Amazon Alexa
Google Home
Apple HomeKit
Works and rated for both Indoor/Outdoor

Belkin WeMo Insight Switch

Belkin's WeMo Insight Switch is an improved version of already simple, Belkin's original programmable WeMo Switch. It is ideal starting for many smart home enthusiasts.

 The Belkin WeMo Insight Switch instantly transforms your standard electrical outlet into a remote controllable one. Through the WeMo App, you can remotely control any electronic device and set schedules. 

A unique advantage of the Belkin WeMo Switch is its integration with both Amazon Echo and Google Assistant, which allows you to control devices plugged into the switch using your voice. Compatible with any 120v wall outlet, the WeMo Switch is easy to set up and use, which is great for anyone who doesn’t want to replace their outlets.

  • Sleek design and easy physical installation
  • Power usage tracking in app
  • Although small it still can block other outlets
  • Advanced configurations needs IFTTT (which means another app)
Belkin Wemo Insight Smart Plug

Wemo Mini Smart Plug

Belkin's WeMo Mini Smart Plug is asolid smart home starting point, and at just $35, the WeMo Mini is the line's best offering yet. It is ideal starting for many smart home enthusiasts.

 The Belkin WeMo Insight Switch instantly transforms your standard electrical outlet into a remote controllable one. Through the WeMo App, you can remotely control any electronic device and set schedules. 

A unique advantage of the Belkin Mini smart plug is its integration with both Amazon Echo and Google Assistant, which allows you to control devices plugged into the switch using your voice. Though it lacks power usage tracking, you may miss it that much.

  • Sleek design and easy physical installation
  • One of the most affordable smart plugs
  • Current does not work Apple Siri (Homekit)
  • Advanced configurations needs IFTTT (which means another app)
Belkin WeMo Mini Smart Plug

iHome iSP8 Wi-FI SmartPlug

iHome iSP8 Wi-FI SmartPlug is smart plug that comes bundled with a remote control which allows you to use the plug without a phone app or even Wi-Fi. The iHome ISP8 SmartPlug works with multiple smart-home platforms, including Amazon Echo, Google Assistant and Apple HomeKit. Plus, its base performance is reliable.

A unique advantage of the BiHome iSP8 Wi-FI SmartPlug is its integration with both Amazon Echo and Google Assistant and bundled remote control. Though it lacks power usage tracking, you may not miss it that much. 

  • Sleek design and easy physical installation
  • Packaged with Remote Control
  • Integration with 3rd Party (Alexa, Google) is limited and can be buggy
  • Does not support IFTTT
iHome iSP8 Wi-FI SmartPlug