MediBang Paint Pro is a free cross-platform paint and drawing tool (a rewrite of FireAlpaca), aimed at manga artists but usable by anyone.
There's a strong core drawing engine with customisable brushes, pressure sensitivity, lots of layer controls and a pile of powerful tools.
The free cloud storage and Windows/ Mac/ iPad/ Android versions mean you really can run the program anywhere.
There's also strong support for collaborating with others, forming groups with other users, working on the same files, and sharing custom tones, brushes and more.
Extras include free downloadable patterns, textures and more, guides for drawing in perspective, and even Voice to Text input.
MediBang Paint Pro has an occasionally quirky interface and it sometimes takes a while to find what you need, but there's plenty to like here, and the ability to work almost anywhere is a real plus.