Instagiffer is a tool which can build animated GIFs from YouTube clips, videos and more.
The process starts by selecting whatever source material you want to use, either by browsing to the relevant file or pasting in the appropriate YouTube URL.
Next, you must define the section of the video which you'd like to export as a GIF. This is horribly awkward here, as there's no timeline, no standard playback controls to locate the right frames: instead you must manually enter the start point and duration as figures, then fine tune to get the results you need. (And even the latter step is tricky as there's no standard "Preview" option.)
Still, you can always take these times by viewing the clip in your regular player. And once you've managed to select the right area, things improve considerably. Setting factors like frame size and playback rate, brightness or quality is as easy as dragging a few sliders; you can add a caption in a click or two, and clicking "Create GIF!" will then build your finished animation.