I considered making a poll for this question, but the field is so completely wide-open to interpreting the classic D&D editions' text that I don't think I can exhaust all the possibilities. Some ideas that come to mind include:
- Ready an action to interrupt other casters (as in 3E).
- Cast a dispel, all enemy spells that round are ruined as they occur.
- Cast a dispel on an area, it lingers until a spell is cast there, ruining it (note that duration in several editions is "permanent").
- Dispels can be used spontaneously whenever an enemy casts a spell (no readying).
- Spells are declared pre-initiative, allowing anyone to react to enemy declarations with "I dispel/counter!"
- No counterspells (it's just a stylistic flourish for standard dispels).
- Other stuff??