Over the past few months I've been doing a bit more "informal" modelling than usual; mostly UML class and sequence diagrams; the sort of thing you want to spend 10 minutes throwing together. The goal of such models is pure communication, when a picture is worth a thousand word email. I really like the more textual browser-based modelling tools that are starting to appear for these purposes:
I'm now doing some slightly more formal modelling and was casting around for a tool with a similar philosophy. For example, I have fond memories of being blown away by TogetherJ when it was first released, and its magical ability to synchronise a set of Java classes with a UML class diagram. Unfortunately subsequent releases seemed to throw a bunch of features that I didn't care about into the tool and it became sluggish and convoluted...
So, this afternoon I was very happy to stumble across UMLet. You can't make an Umlet without breaking a few classes I guess... It is a 5MB download, pure Java, free (GPL license), has a very text-oriented input mechanism, good export options, and some very useful starting diagrams and templates. I spent several hours with it and I am very impressed. The only thing that did not work for me was running it inside Eclipse 3.6, so I used the standalone Swing version.
Which brings me to my question: what modelling tools do you like to use, particularly for quick-and-dirty models? Do you think online (perhaps social?) modelling has a future? Is there a difference in the requirements for a modelling tool designed for a developer/architect than for a Modeller or Business Analyst?