When I was trying free software the program I ended up liking the most was
You must be registered to see the links
The only issue I had with it was because I'm using a cheap Ugee M-1000L the ends of my lines frequently would have a small dot instead of a smooth taper. That issue may have been resolved though, it's been over a year since I used it...
My drawing/editing program I'm currently using is
You must be registered to see the links
, I got lucky and found a physical edition on Amazon for sale at $25... so less than 3 months of a subscription to Creative Cloud for Photoshop. It isn't as robust as Photoshop, but if your style/the style you're aiming for is more drawn vs painted it works fairly well. Not to say I haven't seen some examples that look painted, but you'll probably need to beat the brush engine over the head repeatedly to get a setup capable of doing that reliably.