ShutMeDown is an open-source tool which can automatically shut down, sleep, hibernate or restart your PC.
The program adds an icon to your system tray, and clicking this displays a conventional power menu. There are all the options you need - Lock, Log Off, Sleep, Restart, Hibernate, Hybrid Shut Down, Shut Down - and at a minimum you can carry out any of these in a couple of clicks.
ShutMeDown is really about scheduling, though, and it offers plenty of choice. You're able to schedule a power task to happen at a particular time (5pm tonight), after a period of time has elapsed (in 4 hours from now), or after you've been using your PC for a certain amount of time (you might allow your kids a total of 2 hours use per day, but this can be whenever they like).
The program does its best to minimise any hassles or irritations. An optional desktop gadget warns you about the time left. By default you're alerted 30 seconds before shutdown. It's not forced - if you've unsaved work, you'll be prompted before the system closes or restarts - and if you're doing something urgent then the task can be cancelled.
But it's also possible to take a stricter approach, which might be useful if you're using it as a parental controls-type system. Password-protect the task, choose the "Forced" option, and ShutMeDown will carry out your request at the specified time, no matter what.
(Well, actually a technical user could kill the process or uninstall the program without difficulty, so it's hardly infallible. But if you just want to give someone a prompt that it's time to shut down, this might be enough.)
The interface is cluttered and not always clear, but once you've figured it out, ShutMeDown proves to be a versatile and configurable automation tool.