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As far as I know, there are close to none "enterprise ready" data modelling tools that work on Linux and Mac. I also suspect none are free, whatever the platform. Visio is certainly not one of them.
Why is it so wonderful? Well, to start with, it can retro/forward engineer most JDBC compatible databases. That's a big plus. And it gets better. You can also use it to create a Mondrian schema. Yep. The team at SQL Power published a tutorial for that last week.
I do have to disclose that I will be working on their projects starting in October. I'm not trying to sell it to you; it's free anyways. One thing is for sure though. I can't wait to get my hands in there. So I encourage everyone to grab a copy here and fill as many bug reports as you can. It's not 1.0 yet, so community contributions are a must. Having worked with the team for three weeks back in July, I can guarantee that each and every reported bug and suggested feature is closely studied by the development team.