Crop animated gif image
You can also use: http://ezgif.com/crop?url=http://link/to/image.gif
GIF cropping tool
A simple online tool for cropping and slicing animated gifs.
Just upload your gif and use your mouse or trackpad to select the part of the image you want to crop.
You can also fill in the desired dimensions (in pixels) manually.
If there seems to be any problems with the output file (missing frames/strange artifacts/flickering), try to select the other option under the "Crop with" dropdown. There may be differences, depending on the compression used for the source image. More about this problem.