WebSequenceDiagrams is a site for generating UML sequence diagrams using a mini scripting language.
For example, this:
participant Sync\nService as SS
participant Authorization\nService as AS
participant Resource\nService as RS
activate SS
activate AS
SS->+AS: [1] Request Authorization with X.509 Certificate
AS-->-SS: [2] Return Token
deactivate AS
SS->+RS: [3] Request encrypted data using token
RS-->-SS: [4] Return encrypted data
deactivate SS
Gives you this:
Super cool, and no silly Visio hijinks like this one (Animated GIF):
