Epic Pen is a simple little tool that lets you draw over anything on your screen using a customizable pen. Epic Pen can be considered a straightforward annotation tool as it basically lets you write or draw on top of everything that you can see on your screen, and it's also great at highlighting stuff up in real-time during presentations, tutorials, and so on.
Epic Pen is actually my personal favorite screen marker too as it proved reliable and convenient on every occasion when I needed to use it. It comes with a neat interface which is both easy to use and effective. It makes smart use of simplicity to streamline every supported operation, and it only offers access to features and utilities that are truly needed and not superfluous in any way. For example, there's just a pen and an eraser, no other brushes, sprays, or tools that are likely to be too rarely needed. The size of the pen can be chosen from five presets, and the color from a palette of 16 nuances. Besides the eraser, there's also a handy “Undo” function, and if that's still not even, all the drawings and annotations performed using Epic Pen can be removed from the screen only by pressing the “Clear Screen” button. Taking screenshots (full screen only) and saving them as PNG image files is also possible. Last but not least, every of the available functions can also be accessed via hotkeys. These hotkeys can be customized using the app's “Settings” panel.
Cutting long story short, Epic Pen is a really handy screen marker that also comes with an unbeatable price: it's free (open source). It also comes with multi monitor support and touch compatibility, it's very lightweight and resource-friendly, and it's simply put a great utility that I recommend with all my heart.