I'm the author of a Windows app called Lazy Nezumi Pro.
This app adds input smoothing to almost any art application!
No more switching to other software when you start working on line-art! You can now do more in your favorite application! :)
It has a bunch of cool features like position and pressure processing, but also speed based pressure scaling, pressure curve settings and tilt/rotation smoothing!
It comes with a set of presets so you can start seeing a difference in your lines right away, and you can also create as many of your own hotkey-enabled presets.
You can download the free 30-day full-featured trial here: http://lazynezumi.com
I'm always trying to make it better, so please don't hesitate to send me feedback! :)