Word reference has pulled up these results:

• encadré (for a webpage)

http://www.wordreference.com/enfr/sidebar

This word can also be seen in popular use here: http://www.linguee.fr/anglais-francais/traduction/sidebar.html

I’d say un menu latéral, if looks somewhat like a menu.
But it seems that barre latérale, which sounds maybe more like a toolbar, is nevertheless the most common name for a sidebar.

If for some reason I wanted to avoid barre latérale or plain sidebar, I would probably use “la barre de gauche/droite”, though it loses some generality. If it needs to stay generic, why not “le bloc latéral”?

Or if you want to stay in the tone of entête, you can dig in publishing idiom and say les marges.

Or you could also specify the function of the bar, as in “les flux latéraux” or “le menu latéral”,