For weapons, Base Damage is listed in the weapon descriptions in-game. For spells, it is listed in the spell descriptions in the PDF manual only.
The multipliers -- well, here's a list for weapons:
1.5 -- Short swords, regular bows
2.0 -- Broad swords, longbows, spears, a few unique halberds
2.5 -- Slith spears, halberds, javelins, the Heartstriker bow
3.0 -- Razordisks
For spells, it is listed in the spell descriptions in the same place. From memory,
1.5 -- Bolt of Fire, Icy Rain
2.0 -- Lightning Spray, Divine Retribution
2.5 -- Smite, Divine Fire, Fireblast
3.0 -- Arcane Blow
Edit: Dantius -- you are not using the word "multiplier" to mean the same thing I was above. Since my formula is for averages, "multiplier" just refers to the die size, where for XdY multiplier = (Y+1)/2.