I came across ranking when I was searching for additional systems that could help diversify my existing strategies. Just like my previous post with regard to dynamic portfolio selection through ranking, I found that ranking can also stand by itself as a independent strategy. Most of people will usually refer to it as momentum based systems.
ie. Buying the top x percentile of markets based on the ranking (vice versa for shorts)
Ranking can also be used as entry filters. Over at Au.Tra.Sy, blogger Jez Liberty (a great blog!) did a experiment on a volatility filter based on ranking.
There is so much possibility with ranking that I am going over months of work to see if the fundamental nature of ranking can improve any basic characteristics of my existing trading systems.