The demand for UML online modeling plus the need for web-based collaboration plus the amazing web interfaces that can be created with new technologies (e.g. HTML5 canvas) has created a “new” market for modeling tools: UML online tools. In fact, they are more drawing tools than modeling tools but still, they at least offer you a way to draw and share your (UML mainly but also ER/BPMN) models online.
You can check also this extended and most up to date list of online UML tools
If you want to give them a try (most offer a free plan or at least a free trial), the five tools I’ve been more looking into (let me warn you, don’t expect me to declare a winner, just decide by yourself!) are, in alphabetical order,:
Online diagramming and collaboration. Several modeling languages supported (including UML). For a short introduction see the presentation of this tool for the portal readers in a guest post by Creately authors , see here
Fails in the collaborative aspect and it’s less feature-rich than the others but I love its simplicity. All you see is a empty canvas ready for you to start drawing. Support for BPMN notation.
Support for UML and BPMN diagrams (among many other types). They claim to be “the most widely used online diagramming application”, I wonder how they know that. Collaboration and revision control features available.
HTML5-based. With UML support and real-time collaboration capabilities. It can import Visio files, which is definitely a plus considering that all these tools seem an online version of Visio and are probably targeting the same kind of users.