Inkwash is described as 'Pen-and-ink with living water — a fluid-simulation drawing app in a single HTML file' and is a digital painting tool in the photos & graphics category. There are three alternatives to Inkwash, not only websites but also apps for Windows, Mac and Linux. The best Inkwash alternative is Rebelle. It's not free, so if you're looking for a free alternative, you could try QAquarelle.
Emulates realistic watercolor, acrylic, and dry media with physical paint diffusion, drying control, textured canvas, custom brushes, tilt and pressure support, color blending, masking tools, paint granulation, backruns, stencils, rulers, and high-res export.
Qaquarelle is a very young opensource Qt4-based painting application that is supposed to provide a native way of painting with emulated traditional instruments, including watercolor simulation, full support for tablets and OpenGL-based processing in GPU.
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