BitMappery - the online image editor - has been updated with the all-new timeline mode.
In timeline mode, BitMappery offers tile based animation, where each tile represents a frame in an animated sequence. Each tile can have multiple layers with all editing possibilities of "regular" BitMappery documents.
By using Carbon Copy, a helpful outline is displayed allowing you to hand-draw new frames using the previous frame as reference.
Timelines can be exported as animated GIFs or even sprite sheets (when using one of the pixel art templates).
BitMappery remains free to use and fully open source.