So here are the basic options you can go for to get the wort from boiling to cool enough to add yeast as quickly as possible:
Cooling the Wort in a Water Bath
This really is as easy as it sounds. Obviously if you boil your wort in a metal pot then it will cool a lot quicker due to conduction. If you boil your wort in an electric boiler you will need to syphon off to the fermentation vessel before cooling.
So you sit your vessel in a bath of ice cold water. You can of course throw a load of ice into the water bath to speed up cooling and you will notice after 5 minutes or so the water bath will be noticeably warm. Its a good idea to stir this water to even out the temperature. Also you may find you will need several changes of water to get it to around room temperature.
Immersion coolers are a relatively cost effective solution to cooling your wort rapidly. They are basically a long length of copper pipe coiled many times with a couple of hoses on each end of the pipe. All you need to do is attach one end of the coil to a cold water tap and put the other end in a bucket or in the drain to get rid of the water, turn the tap on low and let it do its thing. You will be surprised at the heat transfer.
It can cools your batch of 21 litres in around 20 minutes to pitching temperature. It has shortened your brew day by a fair amount.
You can find immersion chillers at your home brew shop and a number of people have made their own just by coiling a length of copper pipe. If you have the means to do it then I thoroughly recommend it.
Counter-flow chillers work with the same principle as immersion chillers but in this case the wort is run through a copper pipe that is being cooled from the outside. A common setup is to run a copper pipe through a garden hose. The wort is then run through the pipe whilst cold water is run in the opposite direction through the hose.
Counter-flow chillers have greater cooling effect and the wort can usually be run straight from the boiler to the fermenter and be cool enough to pitch straight away. They are particularly effective when you’re brewing larger batches.