This plugin will create a stroke with a taper, or in other words, a stroke with a start width and an end width, rather than just a constant width. There is an existing plugin that does the same, but this one is a bit faster (although still slow), and has a lot more options.
Settings include start width, end width, exponent (how rapidly the line width changes from start to end), tolerance (there is a inverse relation between speed and accuracy), as well as nearly all the line styles available with the regular GIMP stroke function (e.g. cap/join style).
Be sure to read the readme.txt file for installation and user help.