I want to create some sequence diagrams. Is there a good FLOSS tool for creating good sequence diagrams? Preferably I'd like something that takes some sort of simple text input and spits out a pdf/ps/png/svg. The less manual work I need to do aligning arrows and lines the better.
Bonus points if it's in ubuntu's standard package manager
Take a look at my list of textual UML tools here: http://modeling-languages.com/content/uml-tools (tools in this list are tools that support the use of textual notations/languages to describe UML models and automatically display the corresponding graphical UML diagram).
There are several ones that focus on Sequence diagrams as UMLGraph, WebSequenceDiagram, QuickSequenceDiagramEditor and more.
Graphviz can do UML, apparently. I've not used it for UML, but I have used it for other things, and I've found it easy to write scripts that generate the DOT files to generate graphs.
See http://www.reversejava.com for a dynamic reverse engineering application which generates UML Sequence diagram and view of Participating Class diagram from any Java Application at runtime All you have to do is just run your application and sit back. Reverse Java runs in background tracing all the activities happening inside your application and creates UML diagram for you.