Old way of thinking about yarn: Looking at a pattern and saying, "Oh, it calls for worsted weight yarn...that's all I can use."
New way of thinking about yarn: Shibui Mix suggests mixing it up! Take a fingering weight yarn and a dk weight yarn, use them together to get a worsted weight, or use two light dk weight yarns to equal a worsted weight, and so on. By mixing different yarns and weights to create the recommend weight, you've now open up your yarn possibilities ten-fold. You can also mix fibers and colors to create your own unique look.
To help you get started, ShibuiKnits has created Shibui Mix.
It's a collection of 6 modern, easy to knit and wearable patterns that encourage yarn mixing. They hope you'll use the projects as a launch pad for how you think about selecting yarns. Mix it up!