This might seem like an odd question, but where should I put a sub-menu on the screen, assuming the main menu is horizontal - similar to the Questions/Tags/Users/Badges menu on this site.
The sub-menu uses just contains actions that pertain to the section they are currently navigating within the web app. Should I put it vertically on the left? On the right? If those are the only two reasonable options, I'm torn between having the menu show up first or wanting to put it off to the side so that the interesting content on the page is seen first.
Should I maybe make another horizontal bar below the main one? Just use buttons or make it part of the layout?
Any other options?