Is there any good tool for making Database Design and Prototypes?

I started using NORMA (link fixed Oct 2011) which is useful to capture the relationships and constraints in plain English. See Object Role Modelling too.

This will generate XSDs and SQL scripts.

I hope never to see an ERD ever again...

More for ER than for prototyping (never heard it in the context of databases): If you are on MySQL, try MySQL Workbench. Although it generates MySQL specific code, small adjustments are easily made.

For PostgreSQL, there is a nice list in their wiki, PGDesigner lookes very capable. Furthermore, there will be a Designer in future versions of pgAdmin.

I used in the past the Power Designer (or Q Designer back in those days). It was terrific! Had reverse engineering facilities, script generation.. everything you'd need. But it's not free.

A free tool I've used a bit in the past was DB Designer. But I could use it only for small dbs. Had errors back then working with complicated schema designs, but it was few years ago, so I'm sure it was fixed.

Now I use only SQL Server's own diagramming tool.