If I want to use OpenBox then how can I lock my "monitor" when I leave the PC for a few minutes? Like the gnome-screensaver under GNOME.
The simplest is slock, the suckless screen locker.
You could combine this with xautolock if you wanted to automate it after a period of inactivity.
If you want something more "featurefull" you could install xscreensaver. Of course, gnome-screensaver is an option as well...
I use xlockmore
Install (in fedora):
and I have a keybinding (Ctrl+Alt+l) in my rc.xml file that goes like this: