Short of whipping out a real pen and messing up your LCD, there's no way to quickly draw markings on your screen -- say, to highlight an element on a website or a bullet point in a presentation. That's where a neat utility called Epic Pen steps in, allowing you to creatively deface your desktop... without damaging your display.
Epic Pen weighs in at a couple of megabytes and can be downloaded from SourceForge (note that it's Windows-only). After installation, run the program and a small toolbar will appear on screen, allowing you to select different colours and marking types. Keyboard shortcuts are also available, most of them tied to Control and a number.
By default, the "cursor" mode will be selected, so don't be surprised if you can't draw anything straight away. You'll need to click on the option to its immediate right -- it looks like a slightly larger version of the same icon. Now, if you move the mouse off the toolbar, the cursor will change to a pen and you can hold down the left-mouse button to draw. Be careful with the option on the far right, it acts as an erase-all button.
The design of the program is simple and it's prefect if you just need to highlight on-screen elements quickly. Unfortunately, if you need something more complicated you're out of luck. It can't save drawings or bind them to a particular applications and the ability to add annotations wouldn't go astray.
That said, it works as advertised and other than a lack of instructions, is hard to fault.