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.
Bonsai Proposals - Automated & beautiful proposals for creative freelancers
draw.io - Online diagramming application
Abstract - A secure, version-controlled hub for your design files
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.