Tools for creating Class Diagrams

I used Poseidon UML Community Edition, it's platform independent and makes fine and clean diagrams. There are some screenshots here.

I use GenMyModel, first released in 2013. It's a real UML modeler, not a drawing tool. Your diagrams are UML-compliant, generate code and can be exported as UML/XMI files. It's web-based and free so it matches your criteria.