Rollpie Dot is a free pixel art editor that runs in your browser with no install. Pick pen, eraser, fill, eyedropper, line, rectangle, or select from the tools on the left, then click or drag on the canvas to draw.
Manage layers and color palettes on the right. Zoom with the mouse wheel, hold Space and drag to pan, and press Ctrl+Z to undo. Save your work as a PNG image or a data file from the File panel.