When you want a quick diagram, Visio is indespensable. I used to get along OK with Powerpoint, but Visio gives me more power, without any additional effort.
I use it for:
Flowcharting
Dataflow diagrams
System Architectures
and I just created my first class diagram with it (not a very good one though).
I don't do database design at work. I am in grad school where I have designed databases with Sybase Powerdesigner. This is a really cool tool that can be used to diagram the data entities and then build the DDL (data definition language) SQL to create the tables.