Even though the professionals have discarded it as no longer of significance, there are a lot of retail traders living in the past who still think the Gold/Silver ratio is important. Wheat vs Black Sea is a new curiousity. Silver/Soybean is an old entertainment in the volatility category. Heat/Gas is quite common. Silver/Swiss, and many currency spreads. Interest rate spreads. Cattle/Hogs. Index spreads.

And don't forget, dollars to donuts. : )