Cork! It fits with my neutral/white/pops of color thing I've got going on in my kitchen and living area, and is low-profile enough to not take away from all my pretty plants! The cork naturally absorbs water that might leak out from the soil, saving your shelf. However, I definitely recommend watering your plants in the sink and letting them drain there for a few minutes.
I took a roll of cork (you can get it at pretty much any crafts store), traced and cut out circles, and that was it! I made a few sizes, so you can see some...
...and others are pretty hidden! I couldn't help but hide this little adorable little owl along with it..
Another good flowerpot dish? A teacup and saucer! I just plopped this little guy and his pot inside this teacup and that's it!