I few years ago, I used Data Architect, part of Power Designer from Sybase. It's a little pricy, but it's excellent. And it makes a fine distinction between the physical (SQL) model of data and the ER model of data. It keeps both models side by side.
If you are not too concerned about the difference between SQL and ER, and you just want a diagram, here's something I did once when I needed a diagram in a big hurry.
Crank up MS Access with a new empty database. Set up table links to all the tables in the schema, or just the ones you want to diagram. Use the "Relationships" tool in Access to create lines between the foreign keys and the primary keys they refer to. Classify these relationships as to many-to-many or many-to-one.
In the Access relationships view, move the boxes around until the diagram is pretty. Print.
