In addition to the suggestions as seedling planters, yogurt containers can be used as cutworm collars in the vgetable garden until plants harden off. Cut the bottom off and push the cup into the soil around the plant leaving about 2" exposed above the ground.
They can also be used with lids to store small parts for drip irrigation like connectors and emitters.
In the art studio, they are great for mixing acrylic paints and ceramic glazes. The lids keep mixtures from drying out.