http://www.donationcoder.com/Software/Skrommel/index.html#DimScreen
But as Voldemort said, to do it properly you better use the build-in feature of your laptop for this, and/or do it manually on your monitor. A s those will be true dimming, the software solutions like the program above are just 'simulations'.
related: see the MSDN library for WM_SYSCOMMAND with SC_MONITORPOWER.