Sequence Diagram Editor 

Modeling tool that simplifies and speeds up the creation and maintenance of UML sequence diagrams and telecom/wrieless call flow diagrams. It automatically takes care of layout and formatting (across multiple pages if needed).
sequence diagram uml

EventStudio Sequence Diagram Designer 

EventStudio is a CASE tool for Sequence Diagram, Call Flow, Message Sequence Chart, Use Case and Collaboration Diagram Generation. EventStudio goes beyond UML sequence diagram support and supports advanced constructs for Systems Engineering.
sequence diagram message sequence chart uml interaction diagram use case collaboration diagram context diagram