Polyfiber from Wal-Mart.
It cost like $1.50 for a big honkin 20-oz bag, which will last at least 6 months with weekly changes. I have been using it for years and actually like it better than fiber pads. It keeps the water very clean and by end of the week I have a brown mess. It feels good to be able to just toss it.
I have a filter thatís a little large for my tank so I can afford to stuff as much of it as I can in a chamber without worrying about loss of flow rate.