Software for Sequence Diagrams?

Have a look at UMLet, a Free UML Tool for Fast UML Diagrams which is aimed at fast UML sketching.

_{(source: itmeyer.at)}

For me http://sequencediagram.org is the best. And it's free!

There is www.websequencediagrams.com that is quite cool. I'm having trouble taking it seriously, but it does a really good job at very low effort from my side.

Quick Sequence Diagram Editor might suit your needs. The syntax is strange, but easy to learn. The thing I like about it is that the diagrams are specified using text files, which makes me happy since I don't like the 'pure visual' approach used by the Visio and Rational tools.

The example on the official site may look intimidating but the syntax is actually very simple. Here's a hello world example.

hello_world.sd:

# Put all your Objects here
a:Application.java
c:Config.java

# Put all your Calls between objects here
a:c.readConfigFromFile()