Gimp-Forum.net

While this one I am going to share isn't GIMP,
but I think it's super-useful for quick labeling so I'll share it here: Number-Label your Image
example below. If it's going to save you time, please let me know as I get kick out of that.
It supports touch-screen as well as mouse and drag-able end points and has many arrowhead options.
Example here uses "Custom Text" label-class, but you can set it as Numbers or Letters so that it does 1,2,3...or A,B,C instead of the names shown in example.

Just added ability to add multi-line text (Custom Text).

Added Presets, so that for example you can select "Speech Bubble" preset and it'll set arrowhead and label class to make your labels look like Speech Bubble.