The Singapore Sling was developed by Ngiam Tong Boon for the Raffles Hotel in Singapore in the early 1900's. It is a smooth, slow, sweet cocktail with a complex flavor. That said, this is one of those great cocktails that few people can agree how to make or what exactly goes into it.
The original recipe was almost forgotten by the hotel bartenders and when they wanted to revive the cocktail, the hotel needed to reconstruct the recipe from the bartenders' memories.
This recipe is one of the newer variations. Others can include anything from pineapple to grenadine or other liqueurs like Cointreau. Some of these 'enhancements' have even been adapted by the original bar at Raffles. For example, it is said that the original did not use club soda, though we may never know for sure.
For more background on this cocktail classic, check out Imbibe magazine's recipe: Original Singapore Sling.