Editing Yagura:SRD:Casting Spells (section)

=====Readying to Counterspell=====

You can follow the normal rules to ready an action in case an opponent casts a spell. You can declare your readied action to be a counterspell, in which case the only difference between this and the normal counterspell rules is that an attack of opportunity is not used but your normal actions are.

Of course, you can also cast any other spell as a readied action in order to force your opponent to make a Concentration check, or you can use the ''dispel magic'' spell to counter a spell being cast (see the spell description).