Prickly pear (Opuntia) is a cactus that is hardy even in the cold winters of the Northeastern U.S. and produces delicious fruit in great quantity. Jelly made from prickly pear fruit has fabulous color and taste, but first you've got to deal with the prickles, and then the seeds.
Here are some techniques that make the project fairly easy. The result is absolutely worth it.
If you don't have any prickly pear cacti growing near you, you can sometimes find the fruits for sale in late summer. These have the advantage of having already had the prickles removed. The disadvantage is that you have to pay for them.
Just to give you some idea of quantity, following this method 40-50 prickly pear fruits will yield approximately five 1/2-pint jars of jelly.