While looking for a tool to draw neat sequence diagrams for a new application I developed, I came across this Java program which I found to be useful for drawing sequence diagrams quickly and with minimal waste.
While having a GUI, sdedit is a text-based program when it comes to constructing/editing sequence diagrams. It requires the user to have proper knowledge of sequence diagrams. After learning the basic syntax(it is not very clear just by reading the document in the link posted below) and some hands-on experimentation, it turns out to be a fast tool to use. More importantly, you can keep correcting mistakes and it will not end up messy(compared to my hand-drawn sequence diagrams). Unfortunately, it does not seem to have the function to export the diagram as an image file, but you can get around this by doing a screenshot and then touching up with an image editor like GIMP. Overall, I would say it is a simple and useful program that serves its purpose.