I use a combination of PicMonkey and Serif Digital Scrapbook Artist 2 (and occasionally Paint, because it's actually a pretty useful programme!).
On the left is one of my drawings, which started life as a rather messy scanned image. After a fair bit of work erasing mistakes, fixing broken lines, improving colours and exposure and cutting out I got it to something a bit more like what you would expect in a manga comic. Not an award-winner, but it was pretty rewarding nonetheless.