Since this topic turned up in the multi-paradigm thread, but might deserve a splitting. I found one Erlang debugger which shows a “Process Grid”:
This might not be the ultimate idol, since it is not web based. But still it provides some functionality. Maybe there are even more fancier debuggers around.
Then there is this more academic paper:
CauDEr: A Causal-Consistent Reversible Debugger for Erlang
This reminds me of a functionality that SWI-Prolog has. Namely when you are at an exit or fail port, you can go to the call port, and try everything again. Could this also work in the presence of actors?