I found silver, and while it seems to be used in the Factorization mod, the Silver entry in the Factorization wiki is missing. What's it for?
In Thermal Expansion:
And ofcourse you can pulverize it to get Pulverized Silver which you can combine with Pulverized Gold to make Electrum Blend, which is used to create Electrum Ingots which is used by Thermal Expansion and GregTech.
In Thermal Expansion it's used to make Redstone Energy Cells.
There are a few more uses, just check the wiki or use NEI for it.
It's also used in a number of other mods which the FTB wiki can clarify (I can't post more than two links, unfortunately).