This is a little disc that I picked up at Crate & Barrel. It is described as a candle plate, but I have ended up finding a very good use for it in our home for one very good reason...
Drips. Any water I might have missed while filling the vase gathers where the vase and the coaster meet. And the inch of glass adds a bit of visual integrity to the cheap-o round glass vases I sometimes use.
You can find them here in square or round form on the Crate & Barrel website.
matt at apartment therapy dot com