The official forum is the best place when you're looking for a mod for The Witcher:
link. I've come across other websites hosting some interesting mods:
moddb,
Strategy Informer and
the Witcher's Vault.
I have to say I don't have much experience using mods for the Witcher but here is a short list of the few mods I've tried:
[mod] Stuff that make Geralt's life easier!!-adds three items, one that allows meditation anywhere, one that allows to enable cat vision at will and another one that allows you to access storage anywhere and not just in inns.
Shop buy rate mod-changing prices in game.
[mod] Extreme Immersion Mod-this mod greatly improves the game IMO. It removes most of the hud and makes the game more challenging and also more beautiful. The great thing is that you can add or remove some elements if you don't want to remove everything (simply don't put the texture replacer in your override folder).
There are some other mods that I've been meaning to test for a while now:
[mod] Geralt's Companion Wolf and
[mod] Alchemy Ingredient Icons - Better Color CodingHas anyone tried any of these mods?