The monkey fist knot is very popular, since it makes a spherical paracord knot that can be used in many applications. It is a knot formerly used by sailors, but has been widely adopted because of its unique look.
The monkey fist knot itself is very useful and can be used in anything ranging from a dog collar to a weapon like the bola. As such it is one of the knots you should know. Thankfully it is an easy knot to learn and remember.
For the actual knot you will need some paracord 550, a lighter and optionally a paracord monkey fist jig and something spherical to act as a center (you can use the knot as seen in the following tutorial).
Paracord monkey fist keychain instructions: