Simple ERD right? Why don't you just use a word processor? I understand if you wanted to draw UML diagrams, you'd need a specialized software.
But for ERD? Don't you just draw the 'classes', some lines and their relationship with their multiplicity and so on?
Well, anyway, for my current project, I'm using Visual Paradigm for all my UML diagrams, in case you ever need to make some of those.