Sometimes the easiest way to draw a model is to "write" it down. These tools will render nicely looking UML diagrams from a few lines of text.
A new textual UML tool that allows you to quickly define sequence diagrams online thanks to its autocomplete feature able to guess what UML keywords you're going to write next
Interview with the creator of PlantUML, probably the most popular textual UML tool nowadays. Learn more about why and how the tool was created and its development roadmap