SketchBook Mobile isn’t free, but for serious doodlers, it’ll be well worth the $2. This sketchbook app gives you a range of tools to choose from and has fairly intuitive controls that let you adjust the opacity and thickness of your lines. The app also supports layers for more complicated drawings. This is definitely a sketching app — it doesn’t, for example, respond to the pressure of your finger — but while the app won’t make you an artist, it’s a great tool for letting off some creative steam. There’s also a free version of the application, but the paid version adds in the all-important feature of letting you save your work in a gallery. $1.99 for iOS and Android.