Gliffy.com
With Gliffy online diagramming software, you can easily create professional-looking flowcharts, diagrams, floor plans, technical drawings, and more!
DB Diagram
Auto-create diagram boxes for DB tables and connector lines for their foreign keys.
modelling
WebSequenceDiagrams
http://www.websequencediagrams.com/

Web-Based UML Sequence Diagram

The SD/MSC Generator is an easy alternative to using mouse-centric tools like Microsoft Visio.

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