Every canyoneer of the canyoning party should carry Prusik loops and know how to use them - one short loop for attachment to your harness and a longer foot loop. You may also consider carrying an additional shorter loop which will come in handy if you need to setup a progress capture device for hauling. Prusik loops are also used in a variety of rescue setups and self-belay for abseiling.
A VT Prusik or Valdotain Tresse is another friction knot that is becoming increasing popular in the canyoning world - you can either make your own or buy one such as the Bluewater VT Prusik.
There are of course a number of mechanical ascenders available which make rope ascension so much easier, ranging from ultralight devices such as the Petzl Tibloc which also makes a great progress capture device through to the Petzl Basic and Petzl Ascension. Even if you do carry mechanical ascenders it is always a good idea to carry Prusik loops.
For our full range see: Ascending