My 0.05:
I had an edge during the Imbalance trading time (aka MOCs) that got closed when they made a "continuous" actualization of the imbalance status.
I discovered it making a statistical study of price reaction after imbalance releases, taking the price changes during the release, at the update, 5 min after and at the close, with a note about the market direction that day.
Basically it was front-running. I knew that Imbalance traders would send the majority of their orders (and it was a really big flow) at the update time. So I had 2 baskets ready of both buy and sell imballances, and waited until after the update time the MOC guys started to open their positions. Once I saw they started it, I only had to send the favorable basket orders, wait 5-20 cents until the orderflow ended, and take the profits.
It didn´t gave me amazing results, but it was a steady income with really small risk, that a lot of times turned a negative day into a positive one

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But as I said at the beginning, it died after the 123c rule changes, so it was impossible to catch the right moment, and the strategy just become a "Top short/bottom pick" stuff with the same probability of wining of these (adding the imbalance factor of course, but now it was a too small play for the risk).
It´s an interesting topic. Since the edges are always the result of unconventional thinking, there is a good use of info about old edges. They can help you start thinking in their languaje, and create new edges (my edge was created when I discovered Imbalance trading, and at that time, it was an already done one).