I was looking for a very simple tool to help me put together some sequence diagrams in a hurry, and a Google search turned up sdedit.
This is an Open Source Java-based tool. You download the jar file and run it with the command
java -jar sdedit-4.0-RC1.jar
It looks like a standard visual tool, but you don't actually interact with it visually. You type in statements in the bottom right window. Now that may sound like a retrograde feature, but it's actually very elegant, and you need to try it to realise just how simple it is.
It took me less than 5 minutes to create this PNG image file (generating a PDF or image file is just a couple of button clicks, although the PDF version does suffer some distortion):
The format of the text is really simple, and here are the rules.
It's like a simple program. You first declare all the entities that will be interacting, leave a blank line, then specify all the interactions.
There's one icon for the Actor type, and a standard rectangle for every other. The tool imposes a few simple rules. Undeclared entities are flagged. Only Actors are allowed to initiate interactions, and their interactions are always asynchronous.
It's also possible to mark off sections that are loops or that represent alternate flows. This is one tool that is really worth trying out.
You can use this code to get started:
portal:Process "Sales Portal"
productdb:Database "Product Database"
paymentgw:Process "Payment Gateway"
portal:productdb.get product list
portal:customer.list of products
[c:loop for selected products]
customer:portal.add to shopping cart
portal:customer.redirect to payment gateway
paymentgw:customer.paid - redirect to portal
customer:portal.back to portal
portal:customer.confirmation of order