STLPal

STLPal is a set of free tools for 3D printing files. Resize, rotate, mirror and combine STL parts, fix the errors your slicer complains about (small holes, flipped faces, junk triangles), or convert between STL, 3MF, OBJ, GLB and FBX. You can also turn a photo into a lithophane, push an SVG up into a printable shape, cut an STL into an SVG outline, view STL, 3MF, OBJ, FBX and PLY files, and estimate what a print costs. Everything runs in your browser, so your models never get uploaded.