It's not free. You have to pay 50 bucks for the full version, but, like anyone, we torrent |D I have a cracked version that works with VistaSAI Paint Tool.
That is what I use because it's wonderful for linearting. The feature I like from it is that you can straighten the lines, it automatically straightens the lines for you, or you can use the 'curve line' tool. Has great layering compatibility, tools are more versatile, and it's completely free.