1)   EventStudio Sequence Diagram Designer 2.5
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.

 2)   Trace Modeler for UML Sequence Diagrams 1.0
Trace Modeler is an easy to use and smart editor for UML sequence diagrams... It instantly updates the diagram layout, freeing you to focus on the actual content... Ideal for documenting existing systems and brainstorming new designs.. And fast too!