The Google browser Web Store, "https://chrome.google.com/webstore?hl=en-US" is featuring some of the most fun, easy-to-use, and inspiring artist's software programs I've ever seen. They reside in your browser, but you're able to use them when you're working offline. I've spent several weeks creating and editing many images and photos with them, and I have added some as permanent additions to my software list.
One of my favorites is called, "Harmony". It allows artists to create black and white, or colored drawings, in a new way. The sketches appear to finish their own shadows, and details, as you pull your pen along. You can do it simply, or you can add a "hair" brush, or try the interesting "ribbon" brush. The few combinations of brushes and textures are enough to keep you happily experimenting for a while. The possibilities for line art are amazing.
Draw a line of automatically appearing squares, or draw a line bending around a curve of a face, or an eye, and watch it fill in shadow as it goes. They have thoughtfully posted an introductory video that's quite helpful. I've found the program to be intuitive and easy to feel at home with. Be sure to test this browser add-on out. It's entertaining, free, and gives a whole new slant to what you may create with your graphic pen and a few lines.