Fast Architect helps you create diagrams by using simple intuitive syntax whether you are a developer, an analyst or an architect.
Fast Architect automatically builds a library of components you are using in the diagrams.
Online sequence diagrams from code. Used to be good but now lacks functionality and design. Has interactive diagrams which is cool.
Easy and fast way to create diagrams using custom syntax. Has all the features I need with many different diagram types built in. Contains some interesting features to track your architecture and make it easy to browse.
draw.io - Online diagramming application
PlantUML - PlantUML is an open-source tool that uses simple textual descriptions to draw UML diagrams.
SequenceDiagram.org - Clean and fast online sequence diagram tool using scripting and Drag and Drop
LucidChart - LucidChart is the missing link in online productivity suites. LucidChart allows users to create, collaborate on, and publish attractive flowcharts and other diagrams from a web browser.
Swimlanes.io - Swimlanes.io is a simple free online tool for making sequence diagrams.
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.