I was working with a client this week who needs to get a data modeling tool. I've recently been working with some clients who have their own version of data modeling using Microsoft Visio and powerpoint slides, but this really isn't a good substitute for a true dimensional modeling tool. I've used ERWin in the past at a few clients and it's a great tool, allows one to reverse engineer the database, or better, to create a new model, creating the DDL for the model, and then versioning it for source control. The one problem with ERWin is that is isn't cheap, thus many look to Visio. Visio is a good tool in it's own right, but it's not a true data model tool yet. I suspect this is the direction that Microsoft is trying to go in and maybe with Visio 2010 it will meet that goal.
One tool I was made aware of is Toad Data Modeler. Toad products are made by Quest Software, and I've been working with Toad for Oracle since the beginning. I'm very pleased with the Toad database products, and have already recommended to this client that they buy Toad for DB2, which they have done. I downloaded a version of Toad Data Modeler and my first take of the tool is that it's going to work great for what we need to do, and it's at a very attractive price point as well.
I always stress to clients that they have a printed data model, typically where the developers can see it and visualize the relations between the tables in the model.