A very awesome online tool that lets you create, as if it wasn’t obvious, sequence diagrams online by writing text lines that indicate the elements of the sequence.
The following textual description:
title Authentication Sequence
Alice->Bob: Authentication Request
note right of Bob: Bob thinks about it
Bob->Alice: Authentication Response
creates the following diagram:
As you type into the text area, it automatically changes the diagram. One of the really cool things is that you can have the diagram formatted in a number of ways. I personally love the napkin format shown above.