For once, the German name (Dreieckigentrennungsstäbschen) corresponds to the accepted English term which is "little triangular separating stick thingy". As in, "Mabel, pass fhe little triangular separating stick thingy, would you, there's a luv'.
Not to be confused with the little rectangular separating stick thingies to be found over the border in Marktkauf.
In the States we just call it a divider. I'm guessing Sbrinz is asking because he goes to the same stores I do, where they never have one and it's a pain to keep your stuff separate from the next guy, because the cashier leaves the conveyor running all the time so even leaving a big gap doesn't help the problem.
As slammer says, it's called a Trennstab or to be completely precise a Warentrennstab .
In practice though I suspect you'll more often hear it called just s Stäbli . There don't tend to be that many stick-shaped separating devices in reach of a typical checkout line, so it's fairly unambiguous.
Generally speaking, a Stab is any kind of a stick/wand/bar (cognate to English staff and stave ) and so of course a St ä b li is a small, familiar, or well-liked Stab : a matchstick, chopstick, the post of an earring or what have you.
I'm going to note the term checkout-toblerone, as suggested in the Migipedia aricle below. Or the "Näkubi " has a nice ring to it - (nächster Kunde bitte)
It appears that even among the locals there's no consensus, though Warentrenner is the "offical" term.