Rollpie Mosaic is a free mosaic tool that runs in your browser with no install. Load an image by drag-and-drop or paste, then drag over the area you want to blur to apply a mosaic. Adjust the grid size to change how fine the cells are, and toggle the grid on or off.
Each mosaic stays as a layer you can select, move, resize, and re-granularize. Undo/redo, clear, and Delete-to-remove are supported, and you can save the finished image as a PNG.