Source Intellect
Abbreviated Int
Increases the rate at which you learn weapon skills. (Some people claimed here that it also increases the rate at which you learn other skills, including tradeskills, but Blizzard explicitly states "weapon skills" (source here). Until a source is given for any other claim, we should stick to Blizzard's words.)
Increases your Mana points. Each point of intellect gives you 15 Mana points.
Increases your chance to score a critical hit with spells. For mages, at level 60, 59.5 points of Intellect equal 1% chance to crit with spells. Varies depending on class.
Unlike other spellcasters, Paladins have a base 3.33% chance to crit, which is then increased by Intellect.
A post by Tseric gave the exact figures at level 60 for spell crit for all casters except paladins. They are as follows (post 2.0 paladin information added later):
Int at 60 Int per 1% crit chance \/ Int per 1% crit chance @ 70
Warlock___60.6________________________________81.92
Druid ____60.0________________________________80
Shaman____59.5________________________________80
Mage______59.5________________________________80
Priest____59.2________________________________80
Paladin___54__________________________________80
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Edited, May 9th 2007 5:39pm by Capitolg