this script adds an "anitools" submenu to the "script-fu" menu with entries useful for creating (optimized) animated GIFs:
create scanlines: paint every other horizontal line in each layer with the current foreground layer. this can decrease the size of the resulting GIF and some people like the look :-)
set image framerate: set the frame rate for all layers in the current image.
set layer framerate: set the frame rate for just the current layer.
make GIF animation: rough and unfinished attempt to improve on the "optimize for GIF" plugin.