If you have access to a 3D printer you can turn rubbish into small self-irrigating planters that are perfect for giving your plants a good start.
As 3D printers become more popular and useful, projects are emerging to fit the device into the existing DIY and repurposing subculture. MakerBot Industries site Thingiverse exists to share projects for those who are lucky enough to have access to 3D printers.
If you'd like to make this you'll need the free OpenSCAD plans which you can find on the link below. Print out the planter insert, make a small hole in the bottom and feed a short piece of cord from the bottom of the planter to the bottom of your yogurt cup. Fill the bottom of the yogurt cup with water, plant your seeds in potting soil in the printed planter and cut a hole in the yogurt cup lid to allow the plant access to sunshine. Depending on the plant and your climate this design will take care of watering your plant for up to a week.