Lessons Learned From Executing Successful Virtual Hackathons" (Forbes, ran two recent online hackerthons for the Australian Computer Society. He has written some very useful "These hackerthons ran over a few days. I was one of the mentors fort the first and a lead mentor for the second (mentoring the mentors). The idea is teams are formed and work on a project addressing a specific challenge. There are set deliverables with deadlines, judging, and prizes.
In the past I have helped out with hackerthons at a physical location, ome were national, or global, at multiple locations. An online hackerthon creates new challenges, but also new opportunities. Mostly the online tools mentioned by Steve worked very well, but it is the socialization using them which is important.
A hackerthon is similar in format to a student group projects used in many STEM university courses, except compressed in time: a hackerthon is two or three days, whereas a group project may be ten weeks. There is the potential to use the hackerthon format in industry and in education to give staff and students experience of creative group working quickly.