Magic-user spell, level 1, range 120'
Counter-spell may be cast in reaction to any spell being cast, even if the caster has already taken an action, as long as the enemy caster is within range. Any save against the targeted spell may be re-rolled once if failed. Counter-spell must be cast, however, before the first save is rolled. Counter-spell has no effect against spells that do not grant a saving throw. Though counter-spell is a first level spell, it may be prepared using any level of spell slot. The level of spell slot used grants a bonus to the saving throw re-roll (thus, the extra save granted by counter-spell prepared using a first level has a +1 bonus and counter-spell prepared using a third level slot has a +3 bonus). Scrying magic (such as that provided by a crystal ball or ESP spell) allows a counter-spell to be cast at greater range.
Variation: counter-spell is not a spell, but rather a reaction that any magic-user can take at the cost of a prepared spell, much like how clerics in 3E can substitute a cure spell for any prepared spell. Rules otherwise as above. I can't decide which is better. On the one hand, magic-users are the planning class, and should thus maybe need to plan for counter-spells. On the other hand, spell casting enemies are not that common, so there is a risk that the option would never be taken.
See also this earlier approach to counter-spells.