The dropper loop is probably my most used fishing knot (I suppose it is more that a loop than a knot). Before I head out on the water I have usually tied at least ten of these loops. The dropper loop is used above a sinker to attach a hook and this type of rig is perfect for bottom fishing. Where I come from this is called the paternoster rig and is widely accepted as one of the best reef fishing rigs available.
The dropper loop knot is fairly easy to tie after a bit of practice. I personally prefer to tie a twisted dropper loop as I find that the loops don't tangle with the main line as easily as the hooks sit away from the leader. This loop works well in both monofilament and fluorocarbon leader materials.
How to tie a dropper loop
The first video shows a regular dropper loop and the second demonstrates a twisted dropper loop. I would advice to test both loops out and see which works best for you.