Astropad is a free art app which turns an iPad into a graphics tablet. Simply connect an iPad to a Mac computer via Wi-Fi and then draw. The screens are shared, meaning the larger screen can be used as an editing suite with your software of choice. Astropad works seamlessly with very little lag between the two across both internet and USB connections. There are some features hidden behind a paywall but the free version can be used easily on a Mac without an additional screen.