Others mentioned Observer. That is very cool.

When without a GUI I use the built-in dbg module. Sometimes via the recon_trace wrapper to get the tracing to be rate limited (if on a production cluster).

ttb built-in modules used for tracing on multiple nodes, capture more data and then save to a file and inspect later.

eflame or eep (not the Erlang Proposal, the profiling tool) + kcachegrind to view results.

Erlang has Observer, a built in tool that lets you monitor and trace things as well as well see your process hierarchy.

I'm often not much of a GUI tool guy, but Observer is really cool - the GUI really does give you a nice view of how the supervision tree of a more complex Erlang system is put together.

Yep. Not only that, but I find it to just be insanely fun to go double-clicking with wanton abandon throughout Observer, digging in to everything just because I can. That it's all there, with no setup and effort required, makes it feel almost magical.