DimScreen

DimScreen prevents your device from locking and reduces the screen brightness to the minimum to save battery, so you don't have to be unlocking your device and abusing your power/unlock button over and over if you are reading the web, chatting, listening to music or using any app that doesn't prevent the device from sleeping.

It's the easier way to get an idle screen and a screensaver like behaviour.

You can easily set DimScreen to start automatically on everyboot or use it just for a short time.
You will always be notified while DimScreen is running.

And if you want to lock your device while DimScreen runs, you still can do it using the power button.