NOTE If you want to live on the bleeding edge of this plugin, I'm currently using github (https://github.com/narlynephi/narly_sprite) for my version control.
Narly Sprite is meant to help you create and edit sprite sheets. Pretty simple.
Narly Sprite requires Gimp 2.8+ because it uses group layers to organize the frames in the sprite.
This plugin becomes the most useful when you map the Sprite->Frames entries (Delete Frame, New Frame, Next Frame, Prev Frame) to keyboard shortcuts. I currently have them mapped to ALT= and ALT- for new/delete and ALT, and ALT. for prev and next.
This plugin doesn't actual export anything as animated gifs - you'll have to do
Sprite->Export->Flatten and then export that using the built-in gif exporter.
Below are some screenshots of some of the menus, as well as how the frames are organized into layers:
The images below were all made from the same .xcf file:
Exported as a horizontal sprite sheet:
Exported as a grid sprite sheet:
Flattened and exported as an animated gif:
Narly Sprite also has a settings dialog that lets you choose whether or not the previous frame is displayed with the current frame and with how much transparency:
Previous frame transparency in action: