MotherEffingAnimatedGIF is a very simple-yet-functional and open source web application for quickly creating such images online.
You just drag 'n' drop any number of images to the app, re-order or rotate them, set the image size + quality and the delay between each frame. After that, it is possible to preview the animated GIF and download the output.
Compatibility: All Modern Browsers
Website: http://mothereffinganimatedgif.com/
Download: https://github.com/h5bp/mothereffinganimatedgif/
