Business Letters: Accentuating the Positive
Purdue University OWL
Tips for making business letters more effective by using positive language, even when you must present negative information.
http://owl.english.purdue.edu/handouts/pw/p_subnegmess.html

Interactive Business Letter Generator
Merit Software
A javascript interface takes users through a step-by-step letter writing process.
http://www.businessletterpunch.com/index.html