Ranked in these QuestionsQuestion Ranking
Pro Offline versions available
The downloadable version of Piskel (built with node-webkit) is available for Windows, Linux, and Mac OS X.
Pro Animation support
Piskel includes onion skinning, exporting to sprite sheets or gifs, defining frame rate, and present live playback.
Pro Works in any browser
WPiskel is HTML5-based so it works in any modern browser.
Pro Open source
Piskel is licensed under Apache License, meaning that the user can use the software for any purpose.
Pro Sleek interface
It is also easy on the eyes.
Pro Enough tools to be powerful
It has enough tools to be powerful, but not too many to put off beginners.
Pro Very simple for beginners
Once you get the hang of it its a very powerful tool
Pro Easy to use lighten and darken tool
With the click of a button you can create amazing shading.
Pro Has a straight foreward tile view mode
... so you can instantly see the result while drawing tiles
Con Doesn't work well with drawpads
In the online tool you can't draw lines at all, yet using a graphic tablet, being in the offline version controls are pretty chunky and everytime you start drawing a line, the screen flickers black for a short period of time.
Con Rotation is limited
Rotation is only made in predefined angles. Does not support variable rotations or mouse based rotation. Does not support rotating selected areas, only frames-layers.
Con Shading by hand
Shading can be difficult.
Con Lacks a true resizing tool
There doesn't seem to be any decent tool in Piskel for an easy stretch or shrink. The only ways possible is resizing the whole image or importing and image and shrinking it from the export tool.