I found that near the beginning of the game (where I am now), a nice ice bolt which freezes an opponate for a time then hit them with arrows. Got me through a really tough fight with some orcs east of the archer's guild on the platue.

The spell is Freeze, the effect on a weapon is Frost (which is much more effective, since it ignores the target's resistances).

I'll get that straight one of these days... ; )

But yeah, quite a few mentions of the spell / effect having a powerful, erm, effect throughout the game.