Adobe Line is a surprisingly capable drafting app for drawing straight lines, geometric shapes, perspective views and more. It's designed for use with Adobe's Ink & Slide, but works almost as well as a stand-alone app, and is free to use (you'll only need an Adobe ID).
Launch the app and it loads along with some sample drawings to show what's possible, and easily accessible video tutorials are also on hand. Although you may not need these, as the interface is simple enough. There's a pen, pencil, marker, brush and eraser; a long press on each allows you to customise their size and opacity; and you can start drawing freehand right away.
Life gets a little more interesting with the Touch Slide, Line's digital ruler. Position this on the screen and your next line will run along its edge for perfectly precise results. Then you can shift the slide to draw parallel lines, rotate them, snap lines to endpoints for crisp corners.
Choose another shape, like a circle, triangle, French Curve or rectangle, and Line works in just the same way. You can draw a perfect circle on the screen, say, or expand its size. But you're also able to stretch or distort a shape, maybe morphing a circle into an ellipse.
Easy-to-use perspective tools help you use these options to quickly build complex drawings and sketches. "Stamp packs", pre-drawn sketches of people, plants, furniture etc then help you dress them in just a few seconds.
All this is quite straightforward, at least after you've played with the app for a while. Some helpful new multi-touch gestures simplify your work (swipe two fingers left to undo, right to redo, three fingers to browse history, two finger tap and hold to pan). Creative Cloud integration means you're able to save sketches to your storage space as required. (You only get 2GB in the free offering now, but that should keep you going for a while.)
A surprisingly versatile drafting tool, packed with interesting options.