Google Tag Manager - Google Tag Manager helps make tag management simple, easy and reliable by allowing marketers and webmasters to deploy website tags all in one place.
Dia - Dia is a GTK+ based diagram creation program for GNU/Linux, MacOS X, Unix, and Windows, and is released under the GPL license.
Tealium - Enterprise tag management and digital data distribution (D3P).
draw.io - Online diagramming application
Qubit Opentag - Tag Management
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.