Originally Posted by Little Darwin

There were a few failed attempts at 5 speed indexed shifting, usually on very low end bikes. One phenomenal flop was the Shimano Positron system.

I have seen 6 speed indexed shifters once in a while, used with 6 speed freewheels... but those are pretty old technology, and most 5 and 6 speeds don't have the shaped teeth that really help in indexed shifting.