GamePaint
Try GamePaint
Run GamePaint in your browser here
Works on most platforms including mobile. Requires a browser that supports WebGL such as Google Chrome.
Description
GamePaint lets you draw a game with your own characters, enemies and obstacles. All you need is a mouse or touch device and some imagination. You can assign pre-made behavior to your drawings, turning them into jumping enemies, trampolines, moving platforms, etc. Or you can combine behaviors creating moving trampolines, jumping goal signs or rolling boulders.
You can even share games with your friends, although you have to host the file yourself for now. Hopefully I can create a better system which handles uploading and sharing automatically with a simple share button.
This is in development so give feedback on which features you want me to implement. It would be nice to see the games you are making, share them here if you like. I'm not sure which free hosting is best, but its possible to upload them on GitHub and use the raw link as I have written about in the game description. Maybe I can make a portal where people can upload and play each others games.