I think that counterspelling is an action that is too infrequently used in D&D. It has an obvious benefit, but it is usually too difficult to apply in-game. Counterspelling requires that a caster not cast a spell when he can, that he readies an action, that he identifies the spell being cast, that he knows that spell, and (if a non-spontaneous caster) has that spell prepared.
There are some exceptions to this — dispel magic may always be used as a counterspell (but requires a caster level contest) and the Improved Counterspell feat expands the set of spells that may be used to counter a particular spell. Still, it’s hard to put into practice.
I’d like it to be a little easier, and have designed the Immediate Counterspell feat.
Immediate Counterspell [Spell]
Prerequisites Combat Reflexes, Quicken Spell, Skill Focus (Spellcraft)
Benefit You can counterspell as an immediate action. You must still succeed at a Spellcraft check to identify the spell you are counterspelling. You do not need to use a quickened spell.
Normal You must ready an action in order to counterspell.
This is a fairly potent ability, in that it lets you counterspell out of turn. However, while powerful it is also still a fairly limited ability (you still need to know the right spell and recognize it on casting — or use dispel magic and risk it not working) and the prerequisites don’t otherwise have a lot of synergy between them. This seems fairly well balanced, considering the power it grants.