iShutDown allows you to remotely manage several Macs at once. This solution is comprised of two different apps on two different platforms. First, you have the server application, which you can install on Windows or Mac OS X computers. Then, there is the iOS app, which is the one that lets you control your computers. The server app is free, but the iOS app costs $0.99. It is really affordable, considering that it can let you shut down, restart, sleep, hibernate, or wake up your computer(s).
After you have installed the server app, all you have to do is open the iOS app, which works on iPhones, iPods and iPads, and select your computer from the list of available hosts. Your Mac should be detected automatically. If it is not, make sure you don't have a Firewall that might be blocking it. Windows computers will probably have to also check if Bonjour is installed. Macs have this service installed by default. Then, you click on your computer's IP address and select one of the available options: shutdown, restart, sleep, etc. The app will send the request to your computer and the server app will carry it out.