I've been looking for the same thing. There are a few products out there. Wired switches are easier to find; wireless, self-contained systems are very much harder to find. Here is what I have found so far.
- iBoot: I have one and it really does work. I use it to reboot a home camera when it goes bonkers sometimes. The iBoot is self-contained and has a super-simple web interface for on/off/cycle. You give it an IP address and port on your network, and thereafter, you can access it from anywhere. (To get to it from outside your home network, set up port forwarding on your home router.) I bought a used one off eBay for 50% off the retail price and have had it running for a year now without problems.
NetRC1: Looks like a real product, but they require you to buy a subscription to their web service for accessing your device. Not cool.
BERT 110: Not clear whether this is really available
Wifi, via central controller:
I have not tried any of the wireless solutions.