Marble 1x1 tiles from Lowe's. The cheapest, effective frag plugs I've found so far. Heavy enough they don't fall over, rough enough they don't slide around the rack from moving water, totally inert, corals encrust onto them very nicely.
I tear about a dozen or two off the sheets, soak them in hot water for a few minutes, and 'scrub' the glue off onto a rag. That's why all those plugs were together - had just cleaned the glue off, and they were drying.
bob