If it's leftover from cutting a loaf into bars, I'm with the others -- confetti or rebatch are good choices if you don't mind saving bits and pieces for awhile. I don't care to do that anymore, so here's my preference --
If the soap is fairly fresh so it's still soft, I'll smoosh trimmings and smaller leftovers into a hamburger bun shape and let the "bun" cure along with the rest of the bar soap. I will use it at the sink for hand washing. If the leftovers are too thick to press together, you can grate them or chop them into smaller pieces that will smoosh better. You can also lightly spritz the trimmings with some water to help the soap stick together.
If you're talking about those slivers of soap in the shower (it wasn't clear from the context of your post), I don't use these pieces in new soap. I will stick the old sliver to a fresh bar of soap after either the sliver or the fresh bar is wet from me taking my shower. When the pieces dry, they're usually stuck firmly together.