I'm trying to find a tool where a lot of people can work on a data model collaboratively. Embarcadero has a an ER application called ER/studio which apparently comes with a repository system that acts like typical version control software. That sounds great except ER/studio is expensive and this is a non-profit and open source organization where we encourage members to even contribute small changes.

What's the best solution? Either downloadable software or a web service would work. We don't mind paying, but the cost can't go up with the number of participants...