Pixelart to Multicoloured STL converter     Created by Jayenkai

Note 1 : This won't rescale anything, and has a limit of 512x512 pixels. I may tweak this in future.
Note 2 : To make transparent pixels, either use Alpha channel, or draw in Magenta (255,0,255 / #FF00FF)
Note 3 : Kinda works on mobile, but the export quite obviously fails on iOS (haven't tried it on Android). Use a desktop browser!
Note 4 : Although I do intend to add back the "extrude-length" functionality, it'll mean having to render the pixelart as a 3D object in the little window below, and I don't really fancy tackling that, just yet!!

Thanks to lsv, and @Jerware for the suggestion.