Bukkit

There are 7 current effects:

• Confusion (confuses entities)
• Blindness (blinds entities)
• Real-explosion (normal explosion)
• Fake-explosion (explosion effect but does not damage terrain)
• Real-lightning (strikes entities and does damage)
• Fake-lightning (strikes entities but does no damage)
  
  If you use confusion, you need a confusion-duration line in your configuration (number of seconds the victims are confused for).
  If you use blindness, you need a blindness-duration line in your configuration (number of seconds the victims are blinded for).
  
  Example:

grenades:
  egg:
    radius: 3
    grenade-effect: confusion
    confusion-duration: 10

EDIT: For v1.1:

grenades:
  egg:
    radius: 3
    grenade-effect: 
    - confusion
    - blindness
    confusion-duration: 10
    blindness-duration: 10