draw.io - Online diagramming application
Customer Feedback Portal - Ask customers what you should build next
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Feedback Tools - Curated directory of tools for getting all the feedback
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.
Feedback Fish - Collect feedback & make your customers happy.