It's certainly a first world problem, but finding a way to use a bar of soap once it's become a tiny sliver is tricky dammit. That's why I love this brilliant Stack soap featuring a groove for merging the previous bar.
What's particularly great about the idea is that it's a tiny change to the shape of the bar. It just adds a grooved indent so you can stack the previous bar on top, and after a few uses, they'll be stuck together for good.
And once that merged bar is reduced to a sliver, you can then stack it on top of a new bar again, creating a hygienic circle of life that means you'll never have to discard soap.
At the moment the Stack Soap is seeking funding on Kickstarter so they can create the expensive copper die that shapes the bars. They've already partnered with the company that makes soap for Whole Foods in the US, and with just a quid donation to the cause you're guaranteed at least six bars of the soap once they go into production. So right from the start you'll be saving money. [Kickstarter]