It all started by work of “Prof. Dr. Mathias Weske” from University of Potsdam. He and his colleagues started an open source general purpose web based designer named as Oryx Project.
Orxy uses raphael js library for showing graphics and offers a generous GPL license.
In May 2009 the same team, established Signavio company to enhance and sell the solution. This caused the good oryx prject to be stopped.
Activiti, coming from old JBPM project, was using an eclipse based designer from its first versions. Activiti switched to Signavio probably in 2010 and more or less forgot the eclipse plugin.
After some time activiti modeler development moved to commercial KIS-Project.
Again after some months efforts moved to “Alfresco Activiti” in the form of commercial cloud services.
It is officially stated that Activiti Modelers are not going to be maintained. Only minor donated bug fixes will be merged with the code and actually in the last two years there has not been much activiti around it.
Branched Comunda is using eclipse based designer till today, but they have also started to create their own web based designer, this time from scratch and not depending on orxy. The project name is bpmn.io. It seems to be in its early stages.
PS: There is a good related post here, a little bit old though.