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If you are looking for a simple way to turn your PC on and off remotely, the iShutdown app is a solid option for a small fee. I spent a fair amount of time searching the App Store for a free alternative, but nothing came close in terms of reliability or functionality.
I have exchanged emails with the developer on several occasions, and he is actively improving the app. The progress has been impressive. He recently released a beta version of the server software that can run as a native Windows service. Previously, I had been using the older version and configured it to start automatically with Windows through Task Scheduler. The new beta eliminates that requirement.
At the moment, everything in the iPhone app works well except for Wake on LAN (WOL) over a 3G connection. WOL functions correctly when using Wi-Fi, but that is not my main use case. For now, I am using the iWake app to turn on my machine and iShutdown to power it off.
The developer confirmed that the next update will include the ability to manually enter a MAC address, which should resolve the WOL over 3G issue. He is only waiting for Apple to approve the release. Once that update is available, iShutdown should become a complete and fully functional remote power management solution. I am looking forward to it.
The app is reasonably priced at around 2 to 3 USD in the App Store. If you want an easy way to remotely control the power state of your PC, iShutdown is definitely worth a look.
Update: Hold off on using the beta Windows service version of the server software. It does not appear to function correctly yet. Stick with the standard version for now.