as mcormier said, you can use Xcode's data modeling. There is also Umbrello, which either requires X11 or finding a recent native OS X build. i don't have any

as mcormier said, you can use Xcode's data modeling. There is also Umbrello, which either requires X11 or finding a recent native OS X build. i don't have any experience with it, but also found ArgoUML (http://argouml.tigris.org/ http://argouml.tigris.org/)