Neatbeery has introduced Smoke 1.0 (http://neatberry.com/smoke) for Mac OS X 10.6 and higher. The US$9.99 app offers an unique type of brush that renders graphics with simple mouse movements
You can vary number of parameters to change the brush behavior, color flow, blending mode, background, brush colors and more. This results in the ability to build abstract backgrounds, lighting texts, art-house drawings and more.
Smoke is Universal Binary so runs natively on both PowerPC and Intel Macs. A demo is available for download.
