In the GNU utils,
ps auxf gives the process list with forest (tree).
Is there a equivalent for this command at BSD / OS X systems?
pstree is generally part of the default install or easily installable on bsd systems. That's what I use. For example, you can install it via macports on a mac.
If you really love GNU ps, you can install it onto OSX via one of the big 3 package systems:
pkgsrc (NetBSD) -- my favorite since it works on most popular Unixes
Fink (Debian, but does not share the packageset, and of course does not share the builds)
So far I don't believe OSX has a built in that does this.
But here's an answer I posted on stackexchange as well; a small perl script that determines the process hierarchy and prints it in an indented form using the output of OSX's built-in ps(1).
Tested on OSX 10.6 and 10.9, and should work on linux as well (Sci Linux 6).