Since all these tools lack a validation function their outcomes are just drawings and no better tool for creating nice drawings is a piece of paper and pen. Afterwards you can scan your diagrams and insert them into your team's wiki.
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.
Umbrello UML Modeller is a Unified Modelling Language diagram programme for KDE. UML allows you to create diagrams of software and other systems in a standard format. Our handbook gives a good introduction to Umbrello and UML modelling. http://uml.sourceforge.net/