It's getting a bit late in this thread to help with your immediate need, but since noone mentioned it, or indeed much in specific tools, I'll pitch in.

I assume you mean some kind of UML class diagram, since you didn't specify.

ArgoUML is a free UML diagrammer written in Java, home page gives screenshots etc. I've used it, and consider it pretty good and well-suited to a programmer's needs. - http://argouml.tigris.org/

Only exports to bitmap (eg PNG), SVG or postscript, but frankly, if that is a problem, it's a problem with your word processor or whatever (however you'd fix it is another issue ...). These are sensible, non-proprietary formats for graphics and I can't think of a better format that isn't tied to some application (ODF perhaps shows hope but how many tools, apart from OO.o can read and write .odg files?).

Also there is quite a list at http://en.wikipedia.org/wiki/List_of_UML_tools

Of the ones lists there, I only have experience with Dia (venerable, looks almost like an open-source Visio, but I found it frustrating and gave up); Visio (capable but pricey); and Visual Studio (hate it).

Otherwise, for many diagramming needs I use a sharp pencil, ruler, eraser, and a good scanner. Has been quick and easy and NEVER have to fish around menus or property dialogs looking for some way to achieve what I want. (That said, trying to draw a UML diagram of any size is not for the faint-hearted. Can't beat it on sketches for communicating design concepts, though)

Walter