I'm pretty sure Netflix queues used to work this way. For the DVD-by-mail service, I think. Basically, the web page had a text field next to each movie, and yo

I'm pretty sure Netflix queues used to work this way. For the DVD-by-mail service, I think. Basically, the web page had a text field next to each movie, and you could type in numbers, then submit a form, and it would reorder your queue. The obvious thing to do was swap "1" and "2" to flip the order of the top two items. But you could just set the second one to "0.5" and leave the first at "1". When you submitted the form, it would renumber them all with integers.