In Emacs 23 or later, you can use M-x proced to get a list of running processes. You can sort and filter them in various ways, the most common end goal being to quickly find and kill certain processes in my experience.

However, Proced currently doesn't work on OSX. I just spent a few minutes figuring out why, so I hope writing this could save someone else some effort.

Proced is not based on command line tools such as ps, but uses Emacs Lisp functions implemented in C to get the list of processes (list-system-processes) and to get process attributes (process-attributes). list-system-processes works on OSX, but process-attributes does not. sysdep.c contains a few different implementations guarded by #ifdefs, and this is the one chosen for OSX:

system_process_attributes (Lisp_Object pid)
  return Qnil;

In March 2010, this was discussed on the macosx-emacs mailing list, and a patch giving parts of the needed information yielded a screenshot, but it seems that it wasn't finished, nor merged into Emacs.

Update: I got bored and wrote a patch for it. It basically works, but there are some things that it currently doesn't retrieve, notably memory and CPU usage, and command line arguments.

Emacs' process editor (proced) works on many operating systems, but not OSX. Here is why.