Mudbag is a web-based multi-user database system. It's now in beta, which means that there are two things you need to keep in mind:
Please see the Terms, Conditions, and Warranty before using Mudbag.
With Mudbag you can...
Best of all, mudbag.com service is free up to a limit, and then the costs are very low. The limits are still being worked out, but the free limit will probably allow 2 databases, 10 tables, 100K of data, and 25 emails per batch. There's never a charge for other users to use a database you've created.
Those other users can create databases of their own, too. And then they can share them, and then their uses can create databases, and then... Get the idea?
After the beta period, you'll be able to download the source for Mudbag (it uses PHP and MySQL) and run it on your own system or upload it to your hosting service. It will be Open Source, under the BSD license.
To learn more about Mudbag, read the online help.