I couldn’t resist sharing this tool with you. “FabForce’s DBDesigner 4″:http://www.fabforce.net/dbdesigner4/. Released under the GPL, with versions for Windows and Linux. Yes, *FREE!!!*
I’ve only been using it a for 10 minutes so far, but it seems to be well designed. The interface is a bit clunky when creating tables (I’d have expected to move you from one column to the next, not from the current row to the row below) but I’m sure I’ll get used to it.
And other parts that are confusing are due to my lack of knowledge in this area - I’ve no idea what the difference between the relations 1:1, 1:1 generalization, 1:n, 1:n non identifying and n:m are. All I want to do is show that the user_id column on one table is related to user_id on another 
Documentation is pretty good (OK, outstanding for an Open Source project) and I’m reading that now… will post an update later on when I’m more familiar with it all.