The Emacs fill-sentence macro implements the emacs command fill-sentence, and binds it with the key ALT-j. fill-sentence fills (or justifies) the current sentence without affecting neighbouring sentences. It is very useful if you, like me, like to begin each sentence on a new line, as it enables you to reflow each sentence individually. To use it, simply add it to your .emacs or your .emacs startup file. The code is courtesy of Joe Fineman, and contains many improvements suggested by Christopher L. Conway.  Many thanks to both!