I don't know about anyone else, but I like to save the little soap ends that accumulate around the house and melt them down for new bars. It's just part of my "Reduce, Reuse, Recycle" mantra.
I do something similar with candles, too. When the wick burns way down into a deep wax well, so deep that I can no longer light the wick without burning my fingers, I cut the excess wax walls down, and place the excess wax into the Pyrex bowl which rests on a mug warmer in the bathroom. This gives me a scented, wickless, and flame-free candle made entirely of salvaged wax. But I digress.
Melting soap is a little more involved than one might think. For example, my first attempt ended in disaster! Bringing the old soap up to melting point without burning it is harder than one might think. Trust me, you do NOT want to burn soap in your kitchen. If you know how soap is made in the first place, you can appreciate what I am warning about. You do NOT want to exceed the melting point and turn your kitchen into a rendering works. There are not enough scented candles in your house to cover up that horrible smell.
So gather your old, dried out soaps, and those little hotel soaps you saved, and the little scraps from the bathroom sinks and showers. You need at least a cup of scraps. Grab a muffin pan, a spatula, a sauce pan, and (if you are so inclined) some scented oil. Chop or shave the old soaps into small pieces. Slowly (did I say, 'slowly'?) warm them up until they melt. Do not take your eyes off the soap for an instant. If the soap burns, it's ruined and your house smells like cow hooves.
Then, using a spatula, pour the melted soap into the muffin pan to cool. In an hour you'll have one or more muffin-shaped soap bars. Easy, economical, and good for the environment.
Details here via eHow:
How to Turn Old Soap Pieces Into New Soap Bars
