General Method Description

Summoning is mainly a support skill that helps you earn money in other activities, but there are a couple of methods that use Summoning itself to earn money directly. One of these is using a fruit bat familiar, whose scroll Fruitfall generates a random number of tropical fruit on the ground whenever it is used. This scroll produces from 1 to 6 fruit about 80% of the time it is used; 20% of the time you get nothing. What's particularly nice is that whenever you do get fruit, one of them will be a papaya fruit, which are quite valuable. The others will be a random assortment of pineapples, bananas, oranges, lemons and limes, some of which are worth a fair bit in their own right.

The fruit bat is associated with Karamja, so I like to do this activity at the bank in Shilo Village, but you can really do it anywhere that there's a wide open space near a bank, and few people who will try to grab your fruit if you don't pick them up quickly enough.

Interestingly enough, you don't lose Summoning points while doing this, so you can do this indefinitely, as long as you have supplies.

As an aside, this is also a great method for getting papaya fruit for your own use if you're having trouble buying them on the Grand Exchange.