A step beyond drawing tools that provide you with appropriate shapes for modelling, but do not really “understand” what it is to be a model, are the online modelling tools. The lines between all these categories are a bit fuzzy, be warned.
Some favourites worth considering are
- Umple Online https://cruise.umple.org/umpleonline/ This is a long-established tool for modelling in UML, whose distinguishing feature is that it also uses a parallel textual syntax. This can be particularly useful for conveying the idea that pictures do not have to be less formal than text, but that in fact the same information can be conveyed both ways, with pluses and minuses for each. It is widely used in teaching and has also been used in industry. It has interesting code-generation capabilities and good documentation. Free to use.
- Visual Paradigm Online https://online.visual-paradigm.com (perhaps ought to be in the drawing tool category?)
- PlantText https://www.planttext.com/ Like the desktop application PlantUML https://plantuml.com/ on which it is based, this provides a textual syntax for UML diagrams. Users can “step up” to PlantUML if desired.
- nomnoml https://nomnoml.com/ rather similarly lets users build a variety of UML models from a textual syntax online, exporting as PNG or SVG (with nonnoml syntax embedded for future editing).
- GenMyModel https://www.genmymodel.com exports models as XMI, which can be consumed by EMF-compliant tools such as Epsilon (see https://www.eclipse.org/epsilon/doc/articles/running-epsilon-ant-tasks-from-command-line/#genmymodel and https://twitter.com/EclipseEpsilon/status/1380484362840473610)