This is definitely one of the coolest web apps I have come across in a while. I really like sequence diagrams. However, I rarely want to build out an entire UML model to create one. This easy (and free!) app saves the day!
I can write simple instructions on how a sequence is supposed to work and voila!
Alice->Bob: Authentication Request
note right of Bob: Bob thinks about it.
Bob–>Alice: Authentication Response
There are some nice options for alt, opt, loop and notes. Definitely a good tool for the toolbox!