I have used my pen tablet with SmoothDraw on Windows for a long time and loved it.
What could be a free substitution for it on Mac?
I use SketchBook Express in OS X .The current and upgraded version is Autodesk Sketchbook Pro.
Try these out:
Qaquarelle (this website is in Russian)
Scribbles (free to try, as long as you want)
GIMP Painter (this website is in Japanese)
You can use Google Translate on the foreign language websites.
You might be interested in ArtRage.
Right, you said free. Changed link to the free demo page. There's a 30-day Demo of ArtRage 3 or the free ArtRage 2 Starter Edition.