Check out
LEA Extended Input. It's a new kid on the block, but it looks like it's much more powerful than power grid, and it's under active development.
Last I looked, their sample button graphics/icons were nowhere near as polished as the ones that come with power grid, so it would benefit from a graphic designer cranking out an icon library or something, but it's early days and I intend to create my own custom graphics anyway. (And/or use some of the power grid ones

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The biggest drawback IMHO is that you initially edit/arrange your control panel using the tablet itself instead of on PC like power grid, but according to the forums a PC editor is coming soon. (If LEA lacks a feature you need, post in the forums - the dev(s) seems quite responsive and keen to expand the functionality in ways that people need/request. I got a feature I needed by asking for it

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