Well, not nothing interesting.

I don't think its just the Open source arena, generally I dont know
of any good modeling tools
I only know of three free modeling tools, argouml, tcm and dia.

Dia, seems the easiest to use, and the least sophisticated,
TCM user interface is terrible
Argouml is also very hard to figure out, it seems to try to be smarter than one would want and many features don't work

The main problem I see with models is that you usually want to make models than span an area larger than your screen. Thus printing is necessary, and eventually you throw away the printed paper.

Then you start wanting to include your models in a A4 sized papers to be part of the rest of the design and analysis docs. So you try to take smaller divide your model into smaller part, then the parts and aggregation go out of sync.

Maintaining a set of models, is just hard work, most modeling tools focus more and forcing rules on what you can or can't include in a since models, instead of doing what I want,
composition: glueing ceveral models together to show the overall picture of the design, or information model
decomposition and zooming: allow me to focus on a smaller piece of the model and take pictures of it
Merging: include a model ina work document, a pdf file etc ..., most modeling tools allow to transform a models into a jpg or gif or png, i still mentioned this point to show how important it is, it would be nice thought if the modeling tool can tell me if my jpgs are obosoleted
XML: I would like/trying to write most of my docs, in xml (docbook), it would be nice if I can include few xml lines that will convert into a model, instead of having to draw the models by hand (with the mouse), transform it to a png or gif, and include it as an image in may docbook documen, I think SVG got the potential to solve this problem, but i am not so sure!

Plus, you should really learn more about modeling languages, not all modeling tools support all modeling languages, neither DIA or TCM for example support ORM (the best conceptual information modeling language ever)
Argouml is uml only - and I am not sure how much of uml it support.

Unfortunately, english + some small pictures + an ERD, in 2005 is the best we have (ERD almost directly translate to sql code, this is part why they succeeded)