What's Silver for?

In Redpower:

• With Nikolite in an Alloy Furnace to make Blue Alloy Ingots
• With a stick in a crafting table to make Athame, a dagger strong against Endermen

In IndustrialCraft:

• Used to make Glass Fibre Cables along with glass and a diamond

In Thermal Expansion:

• Used to make a Redstone Transmission Coil used in a Steam Engine
• Used to make a Crescent Hammer along with Iron

In Factorization:

• Used to create Wrathlamps along with Dark Iron Ingots
• Used to make Reflective Mirrors which are used to power Solar Turbines

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.

http://ftbwiki.org/Silver_Ingot
http://feed-the-beast.wikia.com/wiki/Silver_Ingot

Silver is used in Factorization to create reflective mirrors, which power solar turbines, and wrathlamps which are just incredibly bright lights.

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).