Installing smart outlets to automate appliances and holiday lighting is probably the second most common smart device people buy. All of the smart outlets that I have seen connect to HomeKit using Wi-Fi. You have a few different considerations when looking at options for outlets:
Hard Wire or Adapters: You can either remove your existing outlets and re-wire in smart outlets, or you can purchase an adapter that plugs into your existing outlet. You have to physically install the hard wired option, but you get a "zero footprint" smart outlet that doesn't have a thick adapter sticking out of it. Adapters on the other hand are "installed" by simply plugging them in, but result in a thick, unsightly adapter that causes everything to stick out further from the wall.
Size: Watch out for adapters that block one of your outlet's plugs, preventing it from being used.
Number of Controlled Outlets: Some smart outlets have one smart outlet, some have two independently-controlled smart outlets, and some have two outlets on them, but they are both controlled as one.
Energy Monitoring: Some smart outlets monitor the energy use of the devices plugged into it.
PowerStrips: You can purchase smart power strips that have multiple, independently-controlled outlets.
Outdoor: Finally, you can purchase smart outlets designed for use outdoors.
What I Use
I have iDevice Wall Outlets, which are hard-wired into the wall circuits. Each outlet is independently controlled, and provides energy monitoring features.
I also use the iDevice Outdoor Outlet, which has never become unresponsive, despite being outside. It has two outlets, but they are both controlled as one.
Finally I have three different types of plug-in smart outlets:
the Vocolinc SmartBars, which have one outlet, no energy monitoring. It does not block the other wall outlet.
the Satechi Dual Smart Outlet, which has two independently-controlled outlets with energy monitoring. It does not block the other wall outlet.