Create sequence diagrams with very simple text markup. No dragging around shapes, no manually drawing arrows. Just write out the sequence in text and get the nice looking flowchart.
File this under Very cool but may be too cumbersome to use. WebSequenceDiagrams.com allows you to write out a sequence diagram, and it will take your text and create the diagram.
If you need to create sequence diagrams for students, then check it out!