Monofilament. Easily the most popular and used fishing line. It's cheap, durable, and can be bought in, seemingly, any store that has anything to do with fish, which can be tackle shops or a 7-11 that sells Goldfish crackers.
It also is discarded from boats and thrown on shores and riverbanks with reckless abandonment. This Saturday, there will be a wide-spread mono cleanup of the Tampa area. It's the 15th annual Monofilament Cleanup Day, and they never have a shortage of line to gather.