Prospero - Professional Business Proposals
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Proposify - A simpler way to deliver winning proposals to clients.
draw.io - Online diagramming application
Proposalist - A professional, web-based proposal builder.
yEd - yEd is a free desktop application to quickly create, import, edit, and automatically arrange diagrams. It runs on Windows, Mac OS X, and Unix/Linux.