Proposal Writing
The Foundation Center
A short online course in proposal writing, directed at grant seekers but with helpful information for anyone writing a proposal.
http://fdncenter.org/learn/shortcourse/prop1.html

Grant-Writing Tutorial
U.S. Environmental Protection Agency and Purdue University
A comprehensive interactive tutorial on writing EPA grant proposals. Although aimed at a relatively narrow audience, the tutorial provides general information of use to all proposal writers (see especially the Mock Grant Writing Activity).
http://www.epa.gov/seahome/grants/src/grant.htm

How to Write Business Proposals That Persuade Clients and Win Contracts
Klariti
Detailed discuss of proposal writing for business, with strategies for focusing on the client's needs and tips for writing your proposal and evaluating your draft.
http://www.klariti.com/Business%20Writing%20-%20How%20to%20Write%20Business%20Proposals.html
Also see Klariti's Checklist for Writing Business Proposals at
http://www.klariti.com/rfp-business-writing/Checklist-For-Writing-Business-Proposals.html