I've been playing with Dia and been moderately impressed with what I've found. I've been using it as part of my course; being able to export to .pngs is good. It's happy with a wide range of diagrams: mostly I am using it for entity relationship diagrams and UML. There's even a uml to code generator for it which I intend to try using for my Java assignment. While I can't say it's as good as Visio (when did that become part of Microsoft Office), it's a cheap way for students to explore UML and "good enough" for most small projects, and it has an interesting quirky diagram set: isometric game tiles - houses, railways, fields - which can be used to layout a small toy town. It might even be a game prototyping tool one day; with the right tileset..